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.