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 K to O:
K is for knot, a seemingly abstract area of mathematics that has surprising applications in biology.
L is for locus, a word that’s unfamiliar to today’s math majors but was hammered into my head when I was a student. I distinctly remember learning the definition of an ellipse as the locus of points so that the sum of the distances from two fixed points to that point is a constant.
M is for measure, as in “measure theory” behind Lebesgue integration. There’s also a nice discussion of the paradoxical Cantor set that has dimension 
N is for n-tuple, of which the most common type is a vector in 
O is for orthogonal, a synonym for perpendicular.
Mean Green Math 
I am not sure I remember just when I first encountered “locus” as a word. I have some impressions of it appearing in high school, though without much focus. It got to be more common as an undergraduate, and then in grad school it was just everywhere for a couple of courses.