Orthogonality

What is Orthogonality? (Thanks to Andrew Hunt and David Thomas for coining this term in connection with Programming)

When we move parallel to X-axis, no change on Y-axis. In other words, the two lines are independent. In software development, or computing in general, this can be called decoupling. I should be able to change one thing without affecting the other thing.

For example, I should be able to change the UI without having to change the database; or I should be able to change one concrete class without having to change another concrete class.