As Classical Latin developed into Proto-Romance, its lexicon underwent numerous changes.

Irregular nouns and verbs tended to be either regularized or replaced with preexisting regular equivalents. Cf. the loss of edere 'to eat' in favour of manducare or its own regularized compound comedere. Similar motives underlie the general replacement of ferre 'carry' with portare or loqui 'speak' with parabolare and fabulari.^[1]

Various words experienced a significant change in meaning, notable examples being causa ('subject matter' → 'thing'), civitas ('citizenry' → 'city'), focus ('hearth' → 'fire'), mittere ('send' → 'put'), necare ('murder' → 'drown'), pacare ('placate' → 'pay'), and totus ('whole' → 'all, every').^[2]

Certain words may have shed their originally lower-status or humble associations to become default unmarked terms, thus replacing the literary Classical equivalents. Cf. the general loss of equus 'horse' in favour of caballus (originally 'workhorse') or that of domus 'house' in favour of casa (originally 'hut').^[3]

Words that were felt to be too short or phonetically insubstantial were liable to be replaced, often with their own derivatives, hence auris 'ear' and agnus 'lamb' were rejected in favour of their diminutives auricula and agnellus.^[4]

Most Classical particles (such as an, at, autem, donec, enim, etc.) simply died out and survive nowhere in Romance.^[5]

There was a trend towards forming compound prepositions of the type ab ante, which at first simply combined the sense of their constituents (hence the original sense of ab ante was 'from before'). In time many would develop a generic sense, often simply that of one of their constituents (hence ab ante came to mean 'before', in competition with ante). Other examples attested in Late Antiquity are de inter, de retro, de foris, de intus, de ab, and de ex.^[6]

A number of verb-forming (or extending) suffixes were popularized, such as -icare (based on the adjective ending -icus), -ulare (based on the diminutive -ul-), and -izare (borrowed from Greek).^[7]

Numerous foreign terms were borrowed into the Latin vernacular, a majority of which came from Greek, particularly in the domains of medicine, cooking, and Christian worship. A smaller fraction came from Gaulish or Germanic.^[8]

• Appendix Probi
• Reichenau Glossary
• Proto-Romance language
• Phonological changes from Classical Latin to Proto-Romance