Definition
Sign language is an independent natural language that conveys meaning through visual signs: hand shapes and movements, facial expressions, and body movement. It differs from spoken language only in the channel it uses. It has its own grammar and a rich vocabulary, fully capable of expressing thoughts and feelings in fine detail.
How is it different from simple gestures (body language)?
Pointing at a menu in a foreign restaurant and holding up fingers is one thing. A conversation in English with precise grammar rules is on a completely different level. Sign language isn't just improvised miming of what's in front of you.
In sign language, four basic parts combine precisely to form a single word: handshape, location where the hand is placed, movement, and palm orientation. Change the finger angle or position even slightly, and you get a completely different word or a movement with no meaning at all.
Unlike gestures made up on the spot, sign language is an independent language with strict rules and grammar, refined over generations. It also arose and grew naturally within Deaf communities (people who are deaf and use sign language as their first language), and it is a cultural heritage of their own.
You speak with your whole face, not just your mouth
When we talk, we raise our pitch at the end of a sentence or change our tone to show whether it's a question or an exclamation. Sign language has tools that do the same job: facial expression, head angle, and eye gaze.
Linguists call these "non-manual markers," meaning signals that don't come from the hands. In American Sign Language, for example, the same hand movement can mean different things. Raise your eyebrows and it becomes a yes-or-no question. Furrow them and it becomes a who, what, or why question. Nod and it means yes.
Signing with no facial expression is like speaking like a robot, listing words with no feeling or intonation. That's why, in a signed conversation, it matters most to read the facial expression and the angle of the head along with the fingertips.
To be more precise: it isn't a universal language
Many people think sign language is the same everywhere, but the sign language used actually differs completely from country to country. Just as Americans speak English and Koreans speak Korean, Deaf Americans use American Sign Language (ASL), Deaf Britons use British Sign Language (BSL), and Deaf Koreans use Korean Sign Language (KSL).
Even the United States and the United Kingdom, which share spoken English, have different sign languages. ASL was shaped by the influence of French Sign Language, while BSL developed separately in Britain, so their vocabulary and grammar differ a lot. Even the fingerspelled alphabet differs: ASL uses one hand and BSL uses two. So when Deaf people from the two countries meet, they can't simply start chatting.
Sign language is also not a word-for-word hand version of a spoken language. Korean Sign Language, for example, has sentence structure and visual grammar completely different from Korean. In 2016, South Korea passed its Korean Sign Language Act, which officially recognizes it in law as the Deaf community's own language, on equal footing with the national language.
Common misconceptions
Sign language is a universal language that works anywhere in the world.
The sign language differs completely from country to country and culture to culture. Even the sign languages of the US and the UK, which both speak English, differ in grammar and vocabulary.
Sign language is an assistive tool that imitates spoken language with hand movements.
Sign language isn't a tool that depends on spoken language. It is a fully independent language with its own grammar and rich expressive power.
Where you meet it
Making the same hand movement but raising your eyebrows to turn it into a question.
Korean Sign Language (KSL) and American Sign Language (ASL) are separate systems with different words and word order.
In one sentence
Sign language is an independent visual language that fully conveys thoughts and feelings through hand movements and facial expressions.