Updated February 12, 2014.

Definition:

A street slang term for MDMA or Ecstasy.

The term is one of more than 2,300 street terms that refer to specific drug types or drug activity compiled by the White House Office of National Drug Control Policy.
Slang terms can vary from one region of the country to another and even from one neighborhood to another.
Also Known As: Methylenedioxy-methamphetamine

Examples: When they heard him talking about 'Happy Pill' they had no idea that he was referring to the drug MDMA.