1. A screw press for pressing vegetable oil from seeds: In the encyclopaedia there was a photo of an expeller that was used to squeeze the oil from soybeans.
2. A person or something that drives out, drives away, or excludes: The principal, Mr. Straight, was the official expeller when certain students misbehaved badly or who constantly disobeyed the school rules, and he banned them from attending school until further notice.