Terminology
The term "rest mass" + kinetic energy comes from the need to take account of the effects of special relativity on the apparent (or "observed") mass of an electron. It is impossible to "weigh" a stationary electron, and so all practical measurements must be carried out on moving electrons. However special relativity shows that the mass of an object appears to increase as its speed v (relative to the rest frame) increases.
Read more about this topic: Electron Rest Mass