Dictionary.com
definitions

-logy

  1. a combining form used in the names of sciences or bodies of knowledge: paleontology; theology.
  2. a termination of nouns referring to writing, discourses, collections, etc.: trilogy; martyrology.

Origin of -logy

Middle English -logie < Latin -logia < Greek. See -logue, -y3
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for -logy

-logy

n combining form
  1. indicating the science or study ofmusicology
  2. indicating writing, discourse, or body of writingstrilogy; phraseology; martyrology
Derived Forms-logical or -logic, adj combining form-logist, n combining form

Word Origin for -logy

from Latin -logia, from Greek, from logos word; see logos
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
Word Origin and History for -logy

word-forming element meaning "a speaking, discourse, treatise, doctrine, theory, science," from Greek -logia (often via French -logie or Medieval Latin -logia), from root of legein "to speak;" thus, "the character or deportment of one who speaks or treats of (a certain subject);" see lecture (n.).

Online Etymology Dictionary, © 2010 Douglas Harper
-logy in Medicine

-logy

suff.
  1. Science; theory; study:dermatology.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.