Optimisation

Optimisation, in the context of CPU-optimisation, means to arrange what you want to do in the fastest possible manner for the CPU you are using. For example, for all x86 CPUs, if you want to multiply a variable by 8 it is faster to use:

result := variable shl 3;

than it is to use:

result := variable * 8;

Note that almost all compilers nowadays include "optimisers" that will automatically change (blah * 8) into (blah shl 3), and similar simple optimisations.