Other forms: prevented, preventing, prevents
To prevent something is to stop it before it ever starts. A lock prevents theft, a vaccine prevents illness, a fence prevents the dog getting out onto the road.
Two patterns cover almost every use. Prevent a thing: prevent accidents, prevent damage, prevent the spread of a disease. Prevent somebody FROM doing something: the rain prevented us from leaving. British English drops the from as well — prevented us leaving — but 'prevent us to leave' is wrong in every variety of English. The noun is prevention, the adjective is preventive, and something that could have been stopped is preventable; prevent is stronger than stop, because you stop what has already begun and you prevent what has not.
Prevent is Latin praeveniō, prae, 'before', plus veniō, 'to come' — to come before, to arrive first. Middle English preventen still meant to anticipate, to get somewhere ahead of somebody. The modern meaning grew out of that: arriving first in order to block the way.
Proto-Indo-European
*gʷem-
Latin
praeventus
Middle English
preventen — anticipate
English, today
prevent
Built from
From Middle English preventen (“anticipate”), from Latin praeventus, perfect passive participle of praeveniō (“to anticipate”), from prae (“before”) + veniō (“to come”).
Etymology from Wiktionary, CC BY-SA