Diametric describes something having to do with a straight line cutting through a circle, which is probably what your geometry teacher means by diametric. It's also used for two things that couldn't be more different from each other.
Outside of math class, diametric is most commonly used to mean "completely different." So you might say that your sociable, friendly cat is the diametric opposite of your shy dog. Or that your current French teacher, who's solemn and stern, is the diametric opposite of last year's funny, laid-back French teacher. Imagine these diametric opposites standing on opposing sides of a circle. The word comes from diameter and its Greek root diametros, "diagonal of a circle."