Semi-Open File

A file is semi-open for you when you have no pawn on it but your opponent does. It's the natural home for your rooks: they bear straight down the file at the enemy pawn, which can't be defended by a pawn and becomes a target. The flip side of a pawn break or capture that opens a file only for you.

Source for further study: Nimzowitsch, My System, ch. 2.