The Collaborative International Dictionary of English v.0.48:

Spinous \Spi"nous\, a.
   1. Spinose; thorny.
      [1913 Webster]

   2. Having the form of a spine or thorn; spinelike.
      [1913 Webster]

   Spinous process of a vertebra (Anat.), the dorsal process
      of the neural arch of a vertebra; a neurapophysis.
      [1913 Webster]

WordNet (r) 3.0 (2006):

spinous
    adj 1: having spines; "the dorsal fin is spinous" [syn:
           spinous, spiny] [ant: spineless]
    2: shaped like a spine or thorn [syn: acanthoid, acanthous,
       spinous]