The term orthogonality derives from the study of vectors. Most likely you have run across the term in basic trigonometry or calculus. By definition, two vectors in a plane are orthogonal when they are ...

Plot tan, cot, sec and csc on the interval [-2Pi, 2Pi]. (Note that these functions all have vertical asymptotes.) You should know these graphs well. The trigonometeric functions are all periodic, that ...