Note that in modern terminology, the term "rest mass" is no longer preferred. It is now considered cleaner to use the term mass as a constant. The greater difficulty of accelerating as you get closer to the speed of light is then just a different formula for Newton's second law, instead of a change in the mass ("relativistic mass").

This is all equivalent mathematically, but it seems cleaner in terms of terminology rather than saying that the mass of an object depends on the relative speed to the observer.