Backstop and Mainstay

Backstop noun - (baseball) a fence or screen (as behind home plate) to prevent the ball from traveling out of the playing field.

Mainstay and backstop are semantically related. In some cases you can use "Mainstay" instead a noun "Backstop".

Mainstay noun - Something or someone to which one looks for support.

Backstop and mainstay are semantically related. Sometimes you can replace term "Backstop" with "Mainstay", this nouns are similar.