Definitions for water bear
Random House Webster's College Dictionary
Ref: tardigrade (def. 1). 1
Origin of water bear:
A member of the animal phylum Tardigrada.
Origin: Calque of the Wasserbär, used to describe these animals by Johann August Ephraim Goeze in 1773.
any species of Tardigrada, 2. See Illust. of Tardigrada