The Collaborative International Dictionary of English v.0.48:

Nodule \Nod"ule\ (n[o^]d"[-u]l), n. [L. nodulus, dim. of nodus
   knot: cf. F. nodule.]
   A rounded mass or irregular shape; a little knot or lump.
   [1913 Webster]

WordNet (r) 3.0 (2006):

nodule
    n 1: a small node
    2: small rounded wartlike protuberance on a plant [syn:
       nodule, tubercle]
    3: (mineralogy) a small rounded lump of mineral substance
       (usually harder than the surrounding rock or sediment)