In New Latin lexicography, the word for an entry in a dictionary or an encylopedia is vōx, so "see the entry porcus" would be (vidē) sub vōce 'porcus', abbreviated as s.v. porcus. This abbreviation found its way into English as well as other European languages. Note that some sources alternatively interpret this abbreviation as "sub verbō", but based on the use of the latter word this seems less likely, although theoretically I don't see a reason why it couldn't be used in this meaning. vōx in Latin stands for a combination of speech sounds with a certain meaning, regardless of its part of speech. The Oxford Latin Dictionary, which considers only classical usage, lists it as '10 (gram.) A word' (2nd ed., p. 2320).
The first occurrence of sub vōce with roughly that meaning appears to be in Manilius' Astronomica: propriā melius sub vōce notantur ('is better expressed by the native word'). Apart from this, the phrase [tremuit] sub vōce is found in Lucan to mean 'trembled before his words, at his voice', and Lucretius uses sub verbō to mean 'at [her] word, i.e. at once', where verbum means 'command'.