The Collaborative International Dictionary of English v.0.48:

Cnida \Cni"da\ (n[imac]"d[.a]), n.; pl. Cnid[ae]
   (n[imac]"d[=e]). [NL., fr. Gr. kni`dh nettle, sea nettle.]
   (Zool.)
   One of the peculiar stinging cells found in C[oe]lenterata; a
   nematocyst; a lasso cell.
   [1913 Webster]