“entity” is not an official grammar term, but I use it on the site to refer to what we could think of as “mass”.
Primarily, that’s objects and living beings, but it can also be an activity that is grammatically expresses as a thing. Like for example “the reading”.
We can think of entities as the counterpart to verbs. Entities are “mass”, verbs are “motion”.
There are three main word types that represent entities in a sentence – nouns, names and pronouns.