Relation

is basically a set of tuples. For example:


 * A (unary) relation of tall people in my class = {(Amy), (Budi), (Chan)}


 * A (binary) relation of countries with their capital city = {(Afghanistan, Kabul), ..., (Zimbabwe, Harare)}


 * Courses Taken (a ternary relation) = {(Hermione, Potions, A-), (Draco, Potions, B), (Harry, Potions, A), (Ron, Potions, C)}, which can also be represented as a table (see Fig 1).

Trivias

 * There's also a kind of relation called nullary relation, which has no arguments. The possible values to be given are just two: the empty set, i.e. {}, and the set with an empty tuple, i.e. {}. They can be taken as false and true resp.