The Collaborative International Dictionary of English v.0.48:

Phycology \Phy*col"o*gy\, n. [Gr. fy^kos seaweed + -logy.]
   The science of algae, or seaweeds; algology.
   [1913 Webster]

WordNet (r) 3.0 (2006):

phycology
    n 1: the branch of botany that studies algae [syn: phycology,
         algology]