The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. (Zool.) A sting.
      [1913 Webster]