Last summer, Nebus Research had a fun series on the definitions of 26 different mathematical terms, one for each letter of the alphabet. Here are the words from A to E:
A is for ansatz, a uniquely mathematical bit of lingo.
B is for bijection, which I called a one-to-one correspondence when I was a student in the 1980s-1990s. This is a fundamental notion in real analysis and explains why there are exactly as many integers as there are rational numbers, even though the integers are a proper subset of the rational numbers.
C is for characteristic function, which only takes values of 0 and 1. This is similar to an indicator random variable in probability but is different than the characteristic equation encountered in differential equations.
D is for dual, a common notion in graph theory. See also the follow-up post referring to this article on Saving School Math.
E is for error, not in the sense of “a mistake,” but in the sense of the difference between a number and a partial sum of an infinite series whose sum is that number.
Mean Green Math 
I’m embarrassed to be so late acknowledging these. Thanks for paying and bringing some attention to my little blog; I’m flattered.