The root-mean-square value of an alternating current or voltage. It is the square root of the mean value of the squares of the instantaneous values taken over a complete cycle. When an alternating current or voltage is specified, it is almost invariably the root-meansquare value that is used. Also used of quantities that alternate over longer periods, for example, a month or year. Also known as effective value. Abbrev., R.M.S.