Dictionary Definitions

Longman's Dictionary of the English Usage (1981) Viking, 2nd ed.
Websters Third New International Dictionary, 1961

1. study, knowledge, skills, practice (optics, linguistics, paediatrics)
2. systematic formulation; treatise (economics, politics)
3. characteristic activities or behaviour (acrobatics, gymnastics)
4. characteristic qualities, operations, phenomena (acoustics, mechanics, phonetics)

-logy

1. oral or written expression (phraseology)
2. body of writing of or on (a specified subject) (sociology)
3. doctrine or theory;  science (theology, semiology)