Question

What is rest mass?

Answer 1

Rest Mass is a concept from the theory of relativity. One of the consequences of Einstein's special theory of relativity (1905) is that the mass of an object increases with its velocity relative to the observer. When an object is at rest (relative to the observer), it has the usual (inertial = tendency to resist an applied force) mass that we are all familiar with. This is called the 'rest mass' of the object.
Rest mass is the actual mass that an observer will observe when both the observer and body are in the same frame of reference and the body is at rest with respect to the observer.
If the body has some speed with respect to the observer, from Theory of relativity, the observer will observe mass lesser than the rest mass.