What is a bend, sag, or partitioned chamber, in a drain, soil pipe, sewer, etc., arranged so that the liquid contents form a seal which prevents passage of air or gas, but permits the flow of liquids called?

trap — A bend, sag, or partitioned chamber, in a drain, soil pipe, sewer, etc., arranged so that the liquid contents form a seal which prevents passage of air or gas, but permits the flow of liquids. (Webster's 1913.)