A crocodile is any species belonging to the family Crocodylidae (sometimes classified instead as the subfamily Crocodylinae). The term can also be used more loosely to include all members of the order Crocodilia: i.e. the true crocodiles, the alligators and caimans (family Alligatoridae) and the gharials (family Gavialidae), or even the Crocodylomorpha which includes prehistoric crocodile relatives and ancestors. Crocodiles are large aquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. Crocodiles tend to congregate in freshwater habitats like rivers, lakes, wetlands and sometimes in brackish water. They feed mostly on vertebrates like fish, reptiles, and mammals, sometimes on invertebrates like mollusks and crustaceans, depending on species. They are an ancient lineage, and are believed to have changed little since the time of the dinosaurs. They are believed to be 200 million years old whereas dinosaurs became extinct 65 million years ago; crocodiles survived great extinction events. The word crocodile comes from the Ancient Greek (crocodiilos) "lizard", used in the phrase ho crocodiilos ho potamios, "the lizard of the [Nile] river" to refer to crocodiles in the current English sense.

Crocodiilos/crocodeilos itself is described in reference sources as a corruption of croce ("pebbly"), and driilos/dreilos supposedly meaning "worm" although attested only as "(man with circumcized) penis".[4] It is unclear how well supported this analysis is. There are several variant Greek forms of the word attested, including the later form (crocodeilos)[2] found cited in many English reference works.[3] In the Koine Greek of Roman times, crocodiilos and crocodeilos would have been pronounced identically, and either or both may be the source of the Latinized form crocodilus used by the ancient Romans. The form crocodrillus is attested in Medieval Latin.[4] It is not clear whether this is a medieval corruption or derives from alternate Greco-Latin forms (late Greek corcodrillos and corcodrillion are attested).A (further) corrupted form cocodrille is found in Old French and was borrowed into Middle English as cocodril(le). The meaning of croce is explained as describing the skin texture of lizards (or crocodiles) in most sources, but is alternately claimed to refer to a supposed habit of (lizards or crocodiles) basking on pebbly ground. The Modern English form crocodile was adapted directly from the Classical Latin crocodilus in the 16th Century, replacing the earlier form.The use of -y- in the scientific name Crocodylus (and forms derived from it) is a corruption introduced by Laurenti (1768).