There is no single correct answer, and what word you use to describe an entity depends heavily on the question — "It's an entity in which context?".
If you are talking in the context of a programming language; what paradigms are you following? If you take OOP, as an instance, you would describe the entities, which are objects, using classes. If you take Functional Programming, you would use functions to describe the entities.
If you are talking in the context of a database, where entities consist of units — classified things about which data can be stored (users, posts, etc...) — which have relations with other entities; you would use schemas to describe them.
If you are talking in the context of a software as a whole, you would come across units for handling both behavior and data, and you would describe these using domain models.
So, an entity in which context? :-)