Acronym for Root Mean Square.
RMS is the process used to determine the average power output of the speakers over a long period of time. RMS is derived from an equation which produces the most mathematically accurate measure of a speaker's power output. The RMS value is often seen listed as "watts RMS" on product packaging.
The alternative to RMS is PMPO (peak momentary power output) which represents peak output measured in microseconds rather than over a long period of time.
See also PMPO.