A viper is a type of venomous snake known for its long, hinged fangs that fold back against the roof of its mouth when not in use. When a viper strikes, these fangs swing forward and inject venom into its prey. Vipers include familiar species like rattlesnakes, copperheads, and the European adder.

Most vipers have thick bodies, triangular heads, and vertical pupils like a cat's eye. They're ambush hunters, meaning they lie in wait for prey to come close rather than chasing it down. Many use heat-sensing pits near their nostrils to detect warm-blooded animals, even in total darkness. When threatened, some vipers (like rattlesnakes) give warnings before striking, while others remain perfectly still, relying on camouflage.

The word viper can also be used as an insult for a treacherous or spiteful person. If someone calls another person a viper, they mean that person is sneaky and likely to harm others when they least expect it. In this sense, the word captures how these snakes strike suddenly from hiding. You might hear someone say, “I trusted him, but he turned out to be a viper,” when describing someone who betrayed them.