General Definition: 1. The opening and closing of a mouth of a person or animal, such as a bill, or any thing or any action directly or indirectly associated with this, especially a platypus. As a Noun: 1. The hard flat part of the mouth of a bird-like animal, such as a duck, which is wider and flatter than most bird's beaks. English = bill. 2. An amphibious egg-laying mammal of Eastern Australia that has dense fur and a broad bill, tail and webbed feet. English = platypus.