In Java, an entity is a lightweight persistence domain object that represents a table in a relational database, and each entity instance corresponds to a row in that table. An entity is a group of states associated together in a single unit, and on adding behavior, an entity behaves as an object and becomes a major constituent of object-oriented paradigm.

Entities in Java Persistence API (JPA) are POJOs representing data that can be persisted in the database. The persistent state of an entity can be accessed through either the entity’s instance variables or properties, and the fields or properties must be of specific Java language types, including Java primitive types, Java.lang.String, and other serializable types.

Each entity has a unique object identifier, or primary key, which enables clients to locate a particular entity instance. Entities may inherit from superclasses that contain persistent state and mapping information but are not entities themselves.

In Spring JPA, the @Entity annotation is used to specify that a class is an entity.