Definition:Complement


 * Complementary angles: two angles whose measures add up to the measure of a right angle.


 * Complement of Relation: for a relation $\mathcal R$, its complement is all those pairs which are not in $\mathcal R$.


 * Set Complement or Relative Complement: two related concepts: all the elements of a set which are not in a given subset.


 * Logical Complement: In logic and boolean algebra, the negation of a statement or boolean function.


 * Complement of Lattice Element: An in a strong and precise sense incomparable element of an element of a bounded lattice.


 * Complement of Graph: a graph with the same vertex set but whose edge set is all those edges not in that graph.


 * Complements of Parallelograms: The extra bits that need to be added to a pair of parallelograms sharing a line for their diagonals that need to be added to make one big parallelogram.

Linguistic Note
The word complement comes from the idea of complete-ment, it being the thing needed to complete something else.

It is a common mistake to confuse the words complement and compliment. Usually the latter is mistakenly used when the former is meant.