What term is used to describe the increase in mass of an object as it approaches the speed of light according to a stationary observer?

The term used to describe the increase in mass of an object as it approaches the speed of light, from the perspective of a stationary observer, is "relativistic mass." As an object accelerates and approaches the speed of light, its energy increases due to relativistic effects, which in turn leads to an increase in its relativistic mass. This concept is central to the theory of relativity, where the mass of an object is not a constant but is affected by its velocity.

Length contraction refers to the phenomenon where the length of an object is measured as shorter when it is moving at relativistic speeds compared to when it is at rest. Gravitational mass relates to the mass of an object as it contributes to gravitational attraction and is not specifically about relativistic effects. The term mass dilation, while it may suggest a change in mass, is not commonly used in the context of relativistic theory and can lead to confusion, as the prevailing terminology focuses on "relativistic mass" to describe how mass appears to increase with speed.