This has been reduced in size, usually, over time and has less important function in some related organisms than in others.

It's vocabulary question from USA test prep

Vestigial Structures

Explanation:

Trace structures are a rudimentary (or even functionless) version of a body part, but they perform important functions in a closely correlated, or evolutionarily close, species. That is, they are structures that have been reduced in size, usually over time, and have less important function in some related organisms than in others.

The existence of these vestigial structures is strong evidence that evolution occurs in organisms, since this structure, today without much apparent function, may in the past have been extremely important to the ancestors of that species. We take as an example what happens to some vertebrates, such as fish, amphibians, reptiles, birds and mammals.

Vestigial Structures