mass in special relativity

The term mass in special relativity is commonly used by physicists to mean a quantity that does not depend on the observer or the inertial frame used to observe it. However, because the term relativistic mass is also used, this occasionally leads to confusion. The invariant mass of an object (also known as the rest mass, intrinsic mass or proper mass) is an observer-independent quantity that is synonymous with mass. On the other hand, the relativistic mass of an object (also known as apparent mass) increases with its speed and therefore depends on one's frame of reference. The concept of relativistic mass has gradually fallen into disuse in physics since 1950, when particle physics showed the relevance of invariant mass, to the point that relativistic mass is rarely used in 2007 scientific research literature. However, relativity text books of the early 1920s, written by well-respected physicists, made the term "relativistic mass" common in popular discussions and even in textbooks currently in use.
See more at Wikipedia.org...