Is a gambit still a gambit if it's calculated through to the end? I recently gambitted a knight (I call it a gambit as I gave away a knight without being able to take back).
However I had calculated through all the possibilities and five moves or so later I have an extra pawn and a better position. Does this still count as a gambit or doesn't it as I know I can get the material back?
Essentially, what is the actual definition of a gambit?
No, it's not a gambit; a gambit is 'just' a sacrifice which is part of (well-known) opening theory, nothing more, nothing less.
But, technically, what you describe is not even a real sacrifice, but a 'sham sacrifice'.
Wikipedia gives the following definitions:

Real versus sham
Rudolf Spielmann proposed a division between sham and real sacrifices:

In a 'real sacrifice', the sacrificing player will often have to play on with less material than his opponent for quite some time.
In a 'sham sacrifice', the player offering the sacrifice will soon regain material of the same or greater value, or else force mate.  A sham sacrifice of this latter type is sometimes known as a pseudo sacrifice. (Rudolf Spielman, &quot;The Art of Sacrifice in Chess&quot;, 1995, Dover, ISBN 0-486-28449-2)

In compensation for a real sacrifice, the player receives dynamic, positional, or other non-material advantages which he must capitalize on, or risk losing the game due to the material deficit.  Because of the risk involved, real sacrifices are also called 'speculative sacrifices'.

If the name 'sham sacrifice' sounds too negative to you, just call it a 'tactical combination'.