Fill-in and Second-string

Fill-in noun - A person or thing that takes the place of another.

Second-string and fill-in are semantically related. In some cases you can use "Second-string" instead an adjective phrase "Fill-in".

Second-string adjective - Being a replacement or substitute for a regular member of a team.

Fill-in and second-string are semantically related. Sometimes you can use "Fill-in" instead an adjective phrase "Second-string".