This word is very general and can have multiple meanings or interpretations. In math, orthogonal means that two elements are independent. If two vectors are orthogonal, their scalar product is zero. It also means they are not related at all: a change in one of them doesn't affect the other one at all. That's the way we should think about our software.
Changing a module, class, or function should have no impact on the outside world to that component that is being modified. This is of course highly desirable, but not always possible. But even for cases where it's not possible, a good design will try to minimize the impact as much as possible. We have seen ideas such as separation of concerns, cohesion, and isolation ...
