A variable that is not in itself directly relevant, but that serves in place of an unobservable or immeasurable variable.
Example: Per capita GDP is used as a proxy of standard of living. Birth country is used as a proxy for race. And Body Mass Index is a proxy for actual body fat percentage.
Another type of proxy is a proxy war. For example, in Ukraine there is fighting between the Ukrainian army and Russian-backed separatists. Their conflict, is a proxy war between the NATO alliance (the west) who back Ukraine and Russia who back the separatists, who provide them with funding, weapons and training.
Wisdom: “Nature will not be admired by proxy.” — Winston Churchill