Is a heuristic an algorithm

In practice, it may be. But they are still different things.

A heuristic is a rough approximation that can help solving a problem.
For example, in a path-finding algorithm, if your destination is northbound, you may try goring north first. This is the heuristic.

It may not be correct (there may be a dead-end) but most of the time, it helps getting there faster.