In geometry, when two lines are orthogonal if they meet at right angles, such as the axes on a Cartesian plain. Move along one of the axes, and your position projected onto the other doesn’t change.
In computing, the term has come to signify a kind of independence or decoupling. Two or more things are orthogonal if changes in one do not affect any of the others.
In a well-designed system, the database code will be orthogonal to the user interface; you can change the interface without affecting the database, and swap databases without changing the interface.
The Pragmatic Programmer: From Journeyman to Master
by Andrew Hunt, David Thomas