What does "relativistic mass" mean?

The increased mass of an object moving at relativistic speed relative to an observer.

Relativistic mass

Common synonyms include: apparent mass, deprecated term, effective mass, historical term, increased mass, lorentz factor related.

noun

noun ·Rare ·Advanced level

Noun

1
The increased mass of an object moving at relativistic speed relative to an observer. countable, uncountable
2
(physics) the mass of a body in motion relative to the observer: it is equal to the rest mass multiplied by a factor that is greater than 1 and that increases as the magnitude of the velocity increases wordnet