Etymology: Middle English, from Latin complementum, from complēre to fill up, complete, from com- + plēre to fill — more at full
Date: 14th century
a. something that fills up, completes, or makes perfect
b. the quantity, number, or assortment required to make a thing complete <the usual complement of eyes and ears — Francis Parkman>; especially the whole force or personnel of a ship c. one of two mutually completing parts ; counterpart 2. a. the angle or arc that when added to a given angle or arc equals a right angle in measure b. the set of all elements that do not belong to a given set and are contained in a particular mathematical set containing the given set c. a number that when added to another number of the same sign yields zero if the significant digit farthest to the left is discarded — used especially in assembly language programming 3. the musical interval required with a given interval to complete the octave 4. an added word or expression by which a predication is made complete (as president in “they elected him president” and beautiful in “he thought her beautiful”) 5. the thermolabile group of proteins in normal blood serum and plasma that in combination with antibodies causes the destruction especially of particulate antigens (as bacteria and foreign blood corpuscles) II. verb Date: 1602 intransitive verb obsolete to exchange formal courtesies transitive verb 1. to be complementary to <the illustrations complement the text> 2. obsolete compliment
New Collegiate Dictionary. 2001.