A sniffer is someone or something that sniffs, meaning they breathe in through their nose to detect smells. Dogs are natural sniffers: they use their powerful sense of smell to find hidden treats, track scents, or detect dangers. Airport security teams use trained sniffer dogs to find explosives in luggage. These working dogs can smell things humans can't detect at all.

In technology, a sniffer is a computer program that “sniffs out” information by monitoring data traveling across a network. Just as a dog might sniff the air to detect what's nearby, a network sniffer examines the digital information flowing between computers. Security experts use sniffers to find problems or threats in computer systems, similar to how a guard dog might patrol a building.

You might also call someone a sniffer if they're always sniffing because of allergies or a cold. The word can be slightly dismissive in this context. But whether it's a bloodhound tracking a scent through the woods or a cybersecurity tool protecting a computer network, sniffers excel at detecting things others might miss.