The word orthogonal comes from the Greek orthe and gonia, or ``right angle.'' It was originally used as synonym of perpendicular. This is where the use of ``orthogonal'' in orthogonal lines, orthogonal circles, and other geometric terms come from.

In the realm of linear algebra, two vectors are orthogonal when their dot product is zero, which gave rise a generalization of two vectors on some inner product space (not necessarily dot product) being orthogonal when their inner product is zero.

There are also particular definitions on the following entries:

• orthogonal matrices
• orthogonal polynomials
• orthogonal vectors

In a more broad sense, it can be said that two objects are orthogonal if they do not ``coincide'' in some way.