420 is a code-term that is used to refer to cannabis. The term originates from the California penal code section 420, which outlaws the use and possession of cannabis.
The term 420 was popularized by a group of high school students in San Rafael, California in the 1970s. They would meet after school at 4:20pm to smoke cannabis together. The term 420 quickly spread among other cannabis users, and eventually became internationally known.
Today, 420 is celebrated by cannabis users around the world on April 20th. It has become a symbol of the global cannabis culture, and is often used as a rallying cry for pro-cannabis reform movements.