Any of a group of closely related compounds which include cannabinol and the active constituents of cannabis.
In other words, a cannabinoid is a general name given to a group of chemicals that are found in cannabis. The most commonly-known cannabinoids are THC and CBD, but at least 113 have been identified and classified.
Think about fruit. A banana is a fruit, so is an apple, so is an orange. They have some similarities, but they also have differences. Many times when you combine different fruits together, you get a better end product than either individual fruit. "Cannabinoid" is about as general a term as "fruit."