The tonsils are areas of lymphoid tissue on either side of the throat. An infection of the tonsils is called tonsillitis or tonsilitis. Most commonly, the term "tonsils" refers to the palatine tonsils that can be seen in the back of the throat.
Like other organs of the lymphatic system, the tonsils act as part of the immune system to help protect against infection. In particular, they are believed to be involved in helping fight off pharyngeal and upper respiratory tract infections.