-logy is a suffix in the English language, used with words originally adapted from Ancient Greek ending in -λογία (-logia). The earliest English examples were anglicizations of the French -logie, which was in turn inherited from the Latin -logia. The suffix became productive in English from the 18th century, allowing the formation of new terms with no Latin or Greek precedent.
The English suffix has two separate main senses, reflecting two sources of the -λογία suffix in Greek:
Philology is an exception: while its meaning is closer to the first sense, the etymology of the word is similar to the second sense.
-logy versus -ology
In English names for fields of study, the suffix -logy is most frequently found preceded by the euphonic connective vowel o so that the word ends in -ology. In these Greek words, the root is always a noun and -o- is the combining vowel for all declensions of Greek nouns. However, when new names for fields of study are coined in modern English, the formations ending in -logy almost always add an -o-, except when the root word ends in an "l" or a vowel, as in these exceptions: analogy, dekalogy, disanalogy, genealogy, genethlialogy, herbalogy (a variant of herbology), mammalogy, mineralogy, paralogy, pentalogy, petralogy (a variant of petrology), tetralogy; elogy; antilogy, festilogy, trilogy; palillogy, pyroballogy; dyslogy; eulogy; and brachylogy. Linguists sometimes jokingly refer to haplology as haplogy (subjecting the word haplology to the process of haplology itself).
Additional usage as a suffix
Per metonymy, words ending in -logy are sometimes used to describe a subject rather than the study of it (e.g. technology). This usage is particularly widespread in medicine; for example, pathology is often used simply to refer to "the disease" itself (e.g. "We haven't found the pathology yet") rather than "the study of a disease".
Books, journals, and treatises about a subject also often bear the name of this subject (e.g. the scientific journal Ecology).
When appended to other English words, the suffix can also be used humorously to create nonce words (e.g. beerology as "the study of beer"). As with other classical compounds, adding the suffix to an initial word-stem derived from Greek or Latin may be used to lend grandeur or the impression of scientific rigor to humble pursuits, as in cosmetology ("the study of beauty treatment") or cynology ("the study of dog training").