What phenomenon describes the apparent contraction of an object's length in the direction of motion as observed by a stationary observer?

The phenomenon that describes the apparent contraction of an object's length in the direction of motion, as perceived by a stationary observer, is indeed length contraction. This effect is a fundamental aspect of Einstein's theory of special relativity. According to this theory, as an object moves closer to the speed of light, it appears to shorten in length along the direction of its motion from the perspective of an observer who is not moving with the object.

Length contraction occurs because the laws of physics are the same for all observers regardless of their relative speeds. When an object approaches relativistic speeds (close to the speed of light), time appears to pass slower for that object from the perspective of the stationary observer, which factors into how distance is perceived. This results in a measurable contraction of length along the direction of motion.

The other concepts, such as mass dilation and time dilation, relate to different effects observed in relativistic physics, but do not describe the contraction of length. Mass dilation refers to how an object's mass appears to increase with speed, while time dilation involves the differing passage of time as observed from different reference frames. Relative motion is a broader term that encompasses the movement of objects concerning one another, rather than a specific effect like length contraction.