The Oxford English Dictionary defines a constitution as "a body of fundamental principles or established precedents according to which a state or organisation is governed". The Collins dictionary says a constitution consists of "the fundamental principles on which a state is governed, especially when considered as embodying the rights of subjects".
A constitution sets out how all the elements of government are organised and how power is carved up among different political units. It contains rules about what power is wielded, who wields it and over whom it is wielded in the governing of a country.
And, as a kind of deal or contract between those in power and those who are subjected to this power, a constitution defines the rights and duties of citizens, and the devices that keep those in power in check.
In the case of South Africa, these fundamental principles are contained in a single document: the Constitution of the Republic of South Africa 1996. It has been called the "birth certificate" of a new South Africa.
Our Constitution is the most important - or supreme - law of the land. No other law may conflict with it; nor may the government do anything that violates it. This brings us to the crucial topic of constitutional supremacy.
One of the most important features of a constitutional democracy - which is what South Africa has been since 1994 - is constitutional supremacy.
South Africa did in fact have a constitution before the interim Constitution of 1994 and the final Constitution of 1996 (see history of the Constitution). But that "tricameral" constitution - which created separate parliaments for whites, coloureds and Indians but denied blacks a say in government - was not supreme. It was subject to the whims of parliament - in this case, an unrepresentative one.
Parliamentary sovereignty meant that members of parliament could pass any laws they liked - as long as the correct procedure was followed, it didn't matter if the constitution was violated. Back then the courts had no power to scrutinise and overrule any legislation that violated human rights; nor had any such rights been entrenched in that constitution.
But all this changed in 1994, when South Africa became a constitutional democracy and what was then the interim Constitution became the highest source of authority in the land. Now, of course, the final Constitution is superior to parliament and is the yardstick by which all other laws are judged. It applies to all organs of government - including parliament, the presidency, the police force, the army and the public service.
The supreme status of the Constitution is set out early in the text. Section 2 of Chapter 1 - which deals with founding provisions - is a crucial one. It is entitled "Supremacy of Constitution" and says: "This Constitution is the supreme law of the Republic; law or conduct inconsistent with it is invalid, and the obligations imposed by it must be fulfilled."
This means any law that violates the Constitution, or any conduct that conflicts with it, can be challenged and struck down by the courts.
And this is possible because, in a constitutional democracy where power is properly shared by the executive, the legislature and the judiciary, the courts are independent and subject only to the law and the Constitution itself.
This brings us to the doctrine of the separation of powers.
A crucial function of a constitution - and one of the classic features of democracies - is the division of power among the three pillars of government.
Constitutions protect democracy by separating state power into three arms. The legislature (parliament, the provincial legislatures and local councils) makes the laws and monitors the executive; the executive (the president, deputy president and ministers) makes policy, proposes laws and implements laws passed by the legislature; and the judiciary tries cases and administers justice.
The judiciary is unique in that it is not elected, but is independent. This means no one can interfere in the work of the Constitutional Court and the other courts in the country.
In practice this means each arm of the state keeps watch over the power of the others. The courts can judge the actions of the legislature and the executive but cannot pass laws. The legislature can make laws but cannot hand down judgments or take executive action.
It's not easy to keep these in balance. Often the challenge is to ensure that the executive does not wield its authority without being contained by the other branches.
South Africa's Constitution, for example, describes how the government is formed, how it is elected and how it functions. It outlines the government's powers - as well as the limits to these powers - and entrenches the rights of the people. It also sets out how the government will be accountable to the people who elect it.
The Constitution guarantees democracy by giving every person over 18 the right to vote and ensuring one voters' roll for all adult citizens, regular elections and a multi-party system of government. It says parliamentary elections must be held once every five years.
It explains how parliament and other legislatures work, how national and provincial executives are chosen and how the courts work. The Constitution also establishes six independent state institutions to support our democracy.
The Constitution also defines the features of South Africa's sovereignty - its national territory, citizenship and national symbols.
From this it is clear that our Constitution forms the basis of public life in South Africa. But it does more than just describing the nuts-and-bolts of the functioning of the state. Our Constitution also reflects the hopes and aspirations of a nation torn apart by apartheid. And this leads us to the question of what makes a constitution unique.
Constitutions have special origins
A constitution like ours may, at first glance, look just like any other Act passed by the legislature. But it is not an Act of Parliament: it was passed by a special body called the Constitutional Assembly.
Constitutions are unique - for many reasons. Most constitutions emerge out of special circumstances. Sometimes, as in the case of South Africa, they are a product of turmoil, upheaval and even revolution.
For example, most constitutional processes in Africa in the past 50 years have been preceded by struggles against racial domination, colonialism, prejudice and/or abuse. It's no surprise that the quest for democracy, self-determination and human rights forms the backdrop to many modern African constitutions.
And, given South Africa's past, it's not surprising that our Constitution frequently stresses the need to create a society that is "open and democratic", and that it emphasises dignity, justice and equality.
Constitutions reveal a country's hopes and ideals
Linked to the question of origins is one of ideals. A look at the first page of the document reveals that the language used in our Constitution is at times quite emotive - not the dry legal jargon you find in Acts of Parliament.
It's not surprising: the writers of our Constitution were emotionally invested in the process. They were dealing with the hopes of a nation being reborn - but also had to keep one eye on the very real fears created by the decades of despair that had led the country to this juncture.
There's a good example in the preamble: "We, the people of South Africa, recognise the injustices of our past; honour those who suffered for justice and freedom in our land; respect those who have worked to build and develop our country; and believe that South Africa belongs to all who live in it, united in our diversity."
Constitutions may contain a bill of rights
Constitutions and constitutionalism go hand-in-hand with human rights. Rights are often entrenched in a special part of a constitution, called a bill of rights.
Chapter 2 of the 1996 Constitution contains South Africa's Bill of Rights. It is this part of the Constitution that has attracted the greatest interest - and has had the greatest impact on South Africans - in the past few years.
These provisions deal with the rights to equality, human dignity, life and privacy, among others, as well as the freedoms of religion and expression. They also touch on labour relations, children, education and the legal process. See the page on the Bill of Rights for more.
Constitutions are harder to change
The Constitution itself is protected, which means it is more difficult to change it than it is to change a ordinary laws.
Section 74(2) states that bills amending the Constitution require a two-thirds majority in the National Assembly as well as a supporting vote of six of the nine provinces represented in the National Council of Provinces.
Some parts are even more firmly entrenched. For example, a bill amending Section 1 of the Constitution, which sets out the founding values, requires a 75 percent majority.
Constitutions are interpreted in a special way
There is much debate in legal circles about the correct approach to interpreting legislation. Historically, in the case of normal statutes, words were given their plain and literal meaning.
But this is not always appropriate for a text such as the Constitution, which needs to be read with an eye to the values that run through it. A clause in the Bill of Rights even makes this explicit: Section 39(1) says: "When interpreting the Bill of Rights, a court, tribunal or forum must promote the values that underlie an open and democratic society based on human dignity, equality and freedom."
Constitutions tend to be long
Lastly, the Constitution (including its amendments and schedules - the sections that flesh out details such as the design of the flag and the oath the president takes) is well over 100 pages long. It has 14 chapters and contains more than 40 000 words - quite a bit longer than a normal piece of legislation.