MKS: another name for SI (Système International),
the metric system of units. MKS stands for Meters-Kilograms-Seconds.
The term "MKS" is often used to distinguish from "CGS"
(Centimeters-Grams-Seconds), another metric system.
Most physicists use MKS, while some physicists, most astrophysicists, and
chemists use CGS (although these fields also tend to use whatever unit makes
the number a nice couple-digit-number).