What is language?
Language is an arbitrary, conventionalized association between a symbol and a meaning: there’s no necessary connection between the meaning of a word and how it’s represented in language (spoken, signed, or written). This idea comes from Saussure.
If there was a necessary connection between symbol and meaning, we would expect there to be only one possible language. Even for domains where there’s a closer link, such as onomatopoeia and the first words that a baby speaks (often mama, baba, papa, dada since these are easy to articulate), there are still differences cross-linguistically. And for other words, such as dog, chien, perro, languages differ even more.
The conventionalization criterion distinguishes language from other, non-linguistic forms of communication, such as body language and gesture. Two monolingual speakers of English are equally likely to produce similar or dissimilar gestures in describing a given situation (such as a ball rolling down a hill) as a monolingual speaker of English and a monolingual speaker of another spoken language, but two speakers of ASL will produce signs to describe that situation in a way that are systematically similar to each other and different from another sign language such as BSL.
What is grammar?