cray·fish 
(krāfĭsh′) also craw·fish (krô-)
(krāfĭsh′) also craw·fish (krô-)n. pl. crayfish or cray·fish·es also crawfish or craw·fish·es
1. Any of various freshwater crustaceans of the families Astacidae and Cambaridae of the Northern Hemisphere and the family Parastacidae of the Southern Hemisphere, resembling a lobster but considerably smaller. Also called mudbug; also called regionally crawdad.
2. See spiny lobster.
[By folk etymology from Middle English crevise, from Old French crevice, perhaps from Old High German krebiz, edible crustacean; see gerbh- in the Appendix of Indo-European roots.]
Word History: The crayfish, also known as the crawfish, owes its name to a misunderstanding. The actual source of the word may be the Old High German word krebiz, "edible crustacean," or a word related to it. From this Germanic source came Old French crevice, which when taken into English became crevise (first recorded in a document written in 1311-1312). In Old French and Middle English these words designated the crayfish. People began to pronounce and spell the last part of this word as if it were fish, the first fish spelling being recorded in 1555. Because of a variation in Anglo-Norman pronunciation, two forms of the word have come down to Modern English: crayfish and crawfish.