-logy

CIDE DICTIONARY

-logy, suff. [Gr. , fr. lo`gos word, discourse, fr. le`gein to speak. See Logic.].

 A combining form denoting a discourse, treatise, doctrine, theory, science; as, theology, geology, biology, mineralogy.  [1913 Webster]

OXFORD DICTIONARY

-logy, comb. form forming nouns denoting:
1 (usu. as -ology) a subject of study or interest (archaeology; zoology).
2 a characteristic of speech or language (tautology).
3 discourse (trilogy).

Etymology

F -logie or med.L -logia f. Gk (as LOGOS)