To deter someone is to discourage or prevent them from acting. To deter a thing is to stop or limit it.

// The heavy fines aim to deter people from dumping garbage here.

// The device is designed to deter automobile theft.

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"Sergey and other scientists have proposed that, rather than rely on robust and elaborate defenses, certain grasses negotiated a symbiosis—an ecological partnership—with large herbivores. These grasses offered grazers endless fields of tender green leaves that quickly regenerated when shorn. In exchange for this perpetual sustenance, mammoths and other megafauna trampled, ate, and otherwise deterred the grasses' main competitors, such as shrubs and trees, and fertilized the fields with their copious dung." — Ferris Jabr, Becoming Earth: How Our Planet Came to Life, 2024

The word deter is rooted in fear. It was borrowed into English around the mid-16th century from the Latin verb deterrēre, which in turn was formed by combining de-, meaning "from" or "away," with terrēre, meaning "to frighten." Terrēre is also the source of terror, terrible, and even terrific, which originally meant "very bad" or "frightful." These days, you may be deterred by something that frightens you or by something that simply causes you to think about the difficult or unpleasant consequences of continuing. Things, as well as people, can be deterred: the word can also mean "to stop or limit something," as in "policies that aim to deter vandalism."