The English suffix -ology or -logy denotes a field of study or academic discipline, and -ologist describes a person who studies that field. However not every field or study or discipline is an '-ology', for instance the study of childbirth is midwifery and a practitioner is a midwife.
Etymology
The word ology is a back-formation from the names of these disciplines. "-logy" basically means "the study of ____". Such words are formed from Greek or Latin roots with the terminal -logy derived from the Greek suffix -λογια (-logia), speaking, from λεγειν (legein), to speak. The word ology is thus misleading as the 'o' is actually part of the word stem that receives the -logy ending. For example, the bio part of biology stems from Greek βιος (bios), life. This is why some of the words do not end in -ology (such as mineralogy). ...