A sign is conceptually a word in sign language. But, the English word "sign" is often problematic where it has widely multiple meanings while the term "word" is language-specific. Unfortunately, the term "sign" is kind of a poor word for the concept of "word". The ASL sign/word "[see video]" is language-specific, meaning a "signed word" or "sign as in word".
Meaning: a word in a signed language; signing; a visual-spatial modality of language.
Usage: This sign is kind of more likely used for a (signed) word. E.g. What's the sign for [word].
This sign is also used in contexts of "signing" as in talking.
With some inflections, this sign can change nuances of meanings. For example, modifying this sign shows that a hearing person knows some sign language. E.g. "IX CAN SIGN-inflected." Or, inflecting the sign can also indicate a signing level of learners.
This sign has been used as far back as 1913! And likely long before! See the 1913 video clip far below (under the Deaf Culture tidbits).
Variation (typically referred to native and fluent signers).
Usage: This sign is ideally used as in "What's she/he singing?" or glossed as IX SIGN WHAT? when one misses capturing what the signer say (sign). Or, "I don't understand them signing". Proper usage can be subtle in contexts that are not easy to describe in short.
First 100 words.
As you feel more comfortable with the first few hundreds of ASL signs, progress further with your vocabulary and learn signing more.