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 capiability - used for to enable a single task to be spread efficiently across many CPUs
3. Compilation time - used in scripts when percieved responsiveness is vital
4. Flexibility - the time to implement new features

In practise non-trivial projects often find themselves balancing these