420 (pronounced four-twenty) is cannabis culture slang for cannabis consumption, especially smoking around the time 4:20 p.m., and annual cannabis-oriented celebrations held on 4/20, the U.S. notation for April 20.

Five male high school students in San Rafael, California, coined the term as part of their 1971 search for an abandoned cannabis crop, based on a treasure map made by the grower. Calling themselves the Waldos, because their typical hang-out spot "was a wall outside the school", the five students—Steve Capper, Dave Reddix, Jeffrey Noel, Larry Schwartz, and Mark Gravich—designated the Louis Pasteur statue on the grounds of San Rafael High School as their meeting place, and 4:20 p.m. as their meeting time. The Waldos referred to this plan with the phrase "4:20 Louis". After several failed attempts to find the crop, the group eventually shortened their phrase to "4:20", which ultimately evolved into a code-word the teens used to refer to consuming cannabis.

Steven Hager of High Times popularized the story of the Waldos. The first High Times mention of 4:20 smoking and a 4/20 holiday appeared in May 1991 and erroneously attributed the origin of the term to a police code; this and other spurious origin stories became common. The connection to the Waldos appeared in December 1998. Hager attributed the early spread of the phrase to Grateful Dead followers—after "Waldo" Reddix became a roadie for the Grateful Dead bassist, Phil Lesh—and called for 4:20 p.m. to be the socially accepted time of the day to consume cannabis.

Another San Rafael group led by Brad "The Bebe" Bann claims to have originated the term before the Waldos.

April 20 has become an international counterculture holiday based on the celebration and consumption of cannabis. Events typically advocate for cannabis liberalization and legalization. Vivian McPeak, a founder of Seattle's Hempfest, states that 4/20 is "half celebration and half call to action". Paul Birch calls it a global movement and suggests that one cannot stop events like these.

Many marijuana users protest in civil disobedience by gathering in public to smoke at 4:20 p.m.

In 2017, while marijuana legalization was expanding rapidly in the US, cannabis activist Steve DeAngelo notes that "even if our activist work were complete, 420 morphs from a statement of conscience to a celebration of acceptance, a celebration of victory, a celebration of our amazing connection with this plant" which "will always be worthy of celebration".

North American observances have been held at many locations, including: