Originally Posted by Sisaroth
mass increases because the added energy cant be converted into additional speed because it is already at max speed (light speed). Its also what happens in the LHC with the accelerated protons.
"Apparent mass" or as its usually noted "relativistic mass" is dependent on the speed of the frame of reference of the observer relative to the "particle". Invariant mass or the mass of "stuff that's standing still" is the so to say "real mass" of an item. The proton (or a planet) does not get magically more massive as a result of you passing it at near light speed with your frame of reference. Or it passing you at near light speed (when you assume that your frame of reference is "standing still").
The terminology rises from the habit of dividing stuff with the speed of light in calculations and bcos the mass and energy are related concepts in the theory. However, one should keep in mind that "increasing mass" is phenomenon related to the frame of reference and a little green man sitting on the planet that's going at near light speed without acceleration will measure its invariant mass.