The ability to execute more than one task at the same time, a task being a program. The terms multitasking and multiprocessing are often used interchangeably, although multiprocessing implies that more than one CPU is involved.
The terminology used in Mac OS X is symmetric multiprocessing.
Symmetric Multiprocessing, or SMP, is a multiprocessor computer architecture where two or more identical processors are connected to a single shared main memory
This was introduced with Mac OS X 10.0 and the first confirmed announcement of this feature was in July/Aug 2000, along with the announcement of the first dual-processor PowerMac G4 computers.