Which of the following best describes "relativistic speed"?

Relativistic speed is fundamentally characterized by the point at which the effects of relativity, specifically those outlined in Einstein's theories, become significant. This typically occurs when an object's speed approaches a significant fraction of the speed of light. At this threshold, phenomena such as time dilation and length contraction are no longer negligible and can be observed in practical scenarios.

This understanding of relativistic speed distinguishes it from other concepts. Stationary objects are at rest, which means they do not even approach the speeds at which relativistic effects are considered. Similarly, speeds less than the speed of sound are common in everyday experiences and do not invoke the relativistic effects discussed by Einstein. A constant speed, regardless of motion, does not capture the essence of how relativity alters our understanding of time and space at high velocities; relativistic physics demonstrates that observers in different frames of reference may measure time and distance differently. Therefore, the characterization of relativistic speed as the point at which relativistic effects are observable is precise and aligns with the principles of modern physics.