Source : Webster's Revised Unabridged Dictionary (1913)

Aculeus \A*cu"le*us\, n.; pl. {Aculei}. [L., dim. of acus
   needle.]
   1. (Bot.) A prickle growing on the bark, as in some brambles
      and roses. --Lindley.

   2. (Zo["o]l.) A sting.

Source : WordNet®

aculeus
     n 1: a stiff sharp-pointed plant process
     2: a sharp-pointed process especially a sting of a
        hymenopterous insect