The word "but" used as negation

As ruakh and Barrie have pointed out, but is a coordinating conjunction. Logically, it means the same as and, unlike the other coordinating conjunction, or, which is quite different.
The difference between and and but is not logical or even semantic, but rather pragmatic.

If A is true and B is true, both conjoined constructions in the set {A and B, A but B} indicate this fact. However, A but B carries in addition a presumption to the effect that the speaker did not expect B given A, or believed that A's being true would normally contradict B, or was for some other reason surprised that B is true in this context.

There is no special technical term to denote this except, possibly, contrastive. It is not, however, negation, of any sort.

It's sometimes called a contrastive conjunction.

As in many similar contexts, it's a conjunction. More specifically, it's a coordinating conjunction, or a coordinator.