What does "proper time" refer to?

"Proper time" refers specifically to the time interval measured by an observer who is present at both events being considered and is in their own rest frame. This means that proper time is the time experienced by an observer who is not moving relative to the events happening, meaning they are stationary with respect to the events being timed.

In the context of relativity, proper time is an important concept because it serves as an invariant measure of time that is the same for all observers regardless of their relative motion. This contrasts with measurements taken by observers who may be moving at different velocities or in different gravitational fields, where the passage of time can differ due to the effects of time dilation.

The other options suggest different scenarios related to time measurement—such as situations involving high-speed motion or gravitational effects—but do not accurately capture the essence of proper time as defined above.