The term 'drain opening' refers to a HOLE or outfall that allows water to flow out of a drainage system. A drain opening is typically found at the end of a pipe or channel, and it is designed to allow water to escape from the system. The crossword solutions for this term are 'HOLE' (4 letters) and 'outfall' (7 letters). A HOLE is an opening in the ground or in a surface, while an outfall is the point where water flows out of a drainage system.