Sign has several related meanings, all involving communication or indication:

1. A mark, symbol, or object that conveys information or direction. A stop sign tells drivers when to halt. A store sign announces what's sold inside. Road signs guide travelers, and warning signs alert people to danger. Signs use shapes, colors, words, or pictures to communicate quickly, often to people who might speak different languages.
2. Evidence that something exists or is happening. When you see dark clouds gathering, that's a sign of approaching rain. A student yawning repeatedly might be a sign they stayed up too late. Doctors look for signs of illness, like a fever or rash. Scientists search for signs of ancient life in fossils. In this sense, a sign points toward something that isn't immediately obvious.
3. To write your name in your own handwriting, usually to show agreement or approval. You sign your name on a letter, permission slip, or birthday card. Adults sign contracts, checks, and important documents. An author might sign copies of their book for readers.
4. A system of communication using hand gestures, especially sign language used by deaf communities. People who sign can have complete conversations using their hands, facial expressions, and body language instead of spoken words.