Even more generally, two vectors are orthogonal if their inner product is zero.
This is why the word "perpenidcular" is not used, as sometimes your vectors don't really have "directions" in the intuitive sense of the word (e.g. the inner product space of functions).
In mathematics, orthogonal means perpendicular in a geometric-sense (think two vectors) but can also be used in other contexts with a different meaning.