-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
Middle English -logie < Latin -logia < Greek. See -logue, -y3
British Dictionary definitions for -logy

-logy

combining form
1.
indicating the science or study of: musicology
2.
indicating writing, discourse, or body of writings: trilogy, phraseology, martyrology
Derived Forms
-logical, -logic, combining_form:in_adjective
-logist, combining_form:in_noun:countable
Word Origin
from Latin -logia, from Greek, from logos word; see logos
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.).

-logy in Medicine

-logy suff.
Science; theory; study: dermatology.