From Pulsed Media Wiki

Optimization is process of making something to work more efficiently, in one fashion or another.

Performance optimization

Performance orientated optimizations are looking for ways to minimize RAM, CPU, Bandwidth or other computing resource usage.

Usability optimization

Process of optimizing the usability of a program, for example: how easy it is to do certain basic tasks, how much effort is required to do task X.

Other optimization

Optimization can be applied to anything which needs enhancement in efficiency. The name simply changes, for example tuning a car engine fuel injection to work with new bioethanol based fuels is called tuning, which is simply a synonym for optimization, optimization of fuel efficiency and/or power output.