The word complement (with an e in the second syllable, not to be confused with a different word, compliment with an i) has a number of uses. Generally a complement of X is something that together with X makes a complete whole; that supplies what X lacks. The first e in complete and the first e in complement are etymological cognates of each other in a way that is a useful mnemonic for remembering that this is not compliment with an i.

• In painting and optics, complement refers to complementary colors.
• complement (biology) is a group of proteins of the complement system, found in blood serum which act in concert with antibodies to achieve the destruction of non-self particles such as foreign blood cells or bacteria.
• complement (mathematics) (another disambiguation page)
• In traditional music theory a complement (music) is the interval added to another, that is placed on top of another, so that their complete span is an octave, while in musical set theory the complement of a pitch class set are those pitches not included (the pitches needed to form an aggregate).
• In economics, a complement good is a good often consumed together with the good in question.
• phonetic complement
• complementarity (molecular biology) and complementary DNA
• complementary experiments (physics)