-ology

turns a subject into the name of its study or body of knowledge

Spelled -ology, -logy · -logia 'study, account, discourse'

Greek logia meant discourse or study: biology studies life, geology studies Earth, and chronology puts time into an ordered account.

Words built from it

biology — the study of living things
bio- (life) + -logy (study of)
geology — the study of Earth, its rocks, and its history
geo- (Earth) + -logy (study of)
theology — the study of God and religious belief
theo- (god) + -logy (study or discourse)
psychology — the study of the mind and behavior
psych- (mind or soul) + -ology (study of)
chronology — the arrangement of events in time order
chrono- (time) + -logy (account or study)
etymology — the study of word origins and histories
etymo- (true sense of a word) + -logy (study of)