Definition
A typically legal or less harmful substance, such as marijuana, whose use is statistically associated with an increased likelihood of progressing to a more dangerous or illicit drug.
How People Actually Use It
Believed to be Marijuana. But say, if marijuana did not exist, wouldn't that mean people would go directly to harder drugs? So say "if" marijuana is a gateway drug, that's not such a bad thing...
"In a world without marijuana (Gateway drug) Person 1: Hey man, wanna shoot some smack? Person 2: Of course, it's too bad there isn't some magical herb we could smoke instead that slows people like us from getting to the real dangerous shit!!! Person 1 & 2 shoot up."
Source: Urban Dictionary