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Douglas Harper's Etymology Dictionary
cf.

abbreviation of Latin confer "compare" (see confer).

Wiktionary
cf.

adv. (alternative form of cf English)

WordNet
cf.

adv. compare (used in texts to point the reader to another location in the text) [syn: cf, confer, see, see also]

Wikipedia
Cf.

The abbreviation cf. derives from the Latin verb conferre, to bring together (from which confer is the conjugation of the imperative form), while in English it is commonly read as "compare". The abbreviation advises readers to consult other material, usually for the purpose of drawing a contrast. Many usage guides recommend against the common use of cf. to mean "see also."

Such abbreviations appear most frequently in scholarly contexts, such as in academic articles in humanities, physics, chemistry, geology, open nomenclature biology, theology, philology, or in economic or legal writing.

Usage examples of "cf.".

Of course the people actually able to dominate affairs are those who have the most military power, cf. America at the end of the last war.

As he says, this spelling is not used consistently, whether in LotR or in other sources cf. some of the words already quoted: quesse, sinde, nixe.