From The Collaborative International Dictionary of English v.0.48 [gcide]:
Dialectology Di`a*lec*tol"o*gy, n. [Dialect + -logy.]
That branch of philology which is devoted to the
consideration of dialects. --Beck.
[1913 Webster]
From WordNet (r) 3.0 (2006) [wn]:
dialectology
n 1: the branch of philology that is devoted to the study of
dialects