The Collaborative International Dictionary of English v.0.48:

Raindrop \Rain"drop`\ (r[=a]n"dr[o^]p`), n.
   A drop of rain.
   [1913 Webster]

WordNet (r) 3.0 (2006):

raindrop
    n 1: a drop of rain

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "raindrop":
   Scotch mist, bead, blood rain, dewdrop, drizzle, drop, droplet,
   evening mist, fall, flurry, gout of rain, mist, misty rain, mizzle,
   moisture, patter, pearl, pitter-patter, precipitation, rain,
   rainfall, rainwater, sheet of rain, shower, splatter, sprinkle,
   teardrop, unfrozen hydrometeor, wet