The terms animal testing, animal experimentation, animal research, in vivo testing, and vivisection have similar denotations but different connotations. Literally, "vivisection" means the "cutting up" of a living animal, and historically referred only to experiments that involved the dissection of live animals. The term is occasionally used to refer pejoratively to any experiment using living animals; for example, the Encyclopedia Britannica defines "vivisection" as: "Operation on a living animal for experimental rather than healing purposes; more broadly, all experimentation on live animals,” although dictionaries point out that the broader definition is "used only by people who are opposed to such work.” The word has a negative connotation,