What is the definition of a Fixed Base?

Answer 1

Well, the term is used in a lot of industries, so there is going to be a difference depending on whether you are looking for a fixed-base operator, a fixed-base method, a fixed-base index, or a fixed-base router.

However, without using fixed base an an adjective, the definition in general could be "a foundation that does not move."

Of course a base can be a place or platform (as well as other definitions) and fixed can mean more than staying in one place... like repaired. So, it depends on the context, but that is the best generic definition you can do with only that context.