Short for tetrahydrocannabinol, the term THC most often refers to delta-9 THC, a specific type of THC that is the main psychoactive component of the Cannabis sativa L. plant (cannabis). Delta-9 THC is one of the most prevalent of all naturally occurring chemical compounds found in cannabis and responsible for producing intoxicating effects, often referred to as a "high," when consumed. Delta-8 is another type of THC that is present in trace amounts in cannabis.

The federal Controlled Substances Act (CSA) prohibits the possession, use, and sale of cannabis and products derived from cannabis that contain more than 0.3% delta-9 THC on a dry weight basis (marijuana) (21 U.S.C. §§ 802(16) and 812). Cannabis is marijuana if it has a delta-9 THC concentration greater than 0.3% but is hemp and not a controlled substance subject to the CSA if it contains no more than 0.3% THC (see Practice Note, Hemp and the 2018 Farm Bill: Overview). However, cannabis containing more than 0.3% delta-9 THC and certain products derived from cannabis containing more than 0.3% delta-9 THC, including refined THC, are legal for medical or recreational use in some states (see Practice Note, Marijuana State Legal Status Charts: Overview).