Why is the word "complement" used in set theory? Maybe this should have been on the English Exchange, but why do we use the word "complement" in set theory?  If I have:
$$(A \cup B)'$$
Why does "complement" mean everything but the union?  Is it because it is "all the things" that the original operation is not, thus it "completes" it?
I looked at the dictionary and wasn't sure.
Edit:  $(A \cup B)'$ was only an example so I could use the complement mark.  It was picked at random and was only meant to ask the question of what the word meant.  It was not specifically about a union.  I could have probably picked anything that allowed me to use the "tick mark" to indicate complement.  My MathJax is not good and cumbersome for me, so I only used the single example.

"X complements Y" in colloquial English means basically "X has what Y lacks". This is exactly what the complement is in set theory, except that the complement of $A$ also has none of what $A$ has.
That said, my suspicion is that this term actually originates in mathematical French and was borrowed directly from mathematical French into mathematical English. 
