The process of making things better.

Commonly programs are optimised for RunTime efficiency, that is, so they run as fast as possible, but a number of other things may be optimisation targets can also be used, including:

1. Object code size - used mainly for frequently downloaded Java/JavaScript and to improve cache performance.
2. Threading capability - used to enable a single task to be spread efficiently across multiple CPUs.
3. Compilation time - used in scripts when perceived responsiveness is vital.
4. Flexibility - the time to design/code new features.
5. Portability - run on as many platforms as possible (requires avoiding platform-specific optimisation techniques).

In practise non-trivial projects often find themselves balancing these.