(n) a habit-forming drug that is not addictive but its use may lead to the use of other addictive drugs, Example:one college athlete recently called beer a gateway drug for young people
The Collaborative International Dictionary of English (GCIDE) v.0.53 Collaborative International Dictionary (GCIDE)
n. A passage through a fence or wall; a gate; also, a frame, arch, etc., in which a gate in hung, or a structure at an entrance or gate designed for ornament or defense. [ 1913 Webster ]