Grammaticality
Saying that a sentence is grammatical refers to the sentence being correct, making sense. Ungrammatical means it is isn't.
It is most related to sentence structure - syntax, omission, whatnot. Many linguists may also consider something ungrammatical if it makes no semantic or pragmatic sense.
Keep in mind that the majority of discourse (spoken language) is ungrammatical in its fragmented nature,
which is not necessarily considered horrible by us problem-resolving humans.
We barely even notice.
In this sense, grammaticality is intuitively a relative measure, though literature tries to keep it as black and white and technical as sensible.
On that relative note, this can also be about rules that appear or disappear.
For example, you can say that the use of 'whom' used to be more grammatical that it is now: Whom used to have stricter rules, but 'who' is now also considered correct almost everywhere where previously only whom was correct.
This extends the above meaning with langauge change (and arguably preference).
You can also say that something carries grammatical information (cf. lexical information, semantic information, etc.).