What is the difference between an entity and a deity?

There is One Deity and He is the perfect, ever-existing, omnipotent, omnipresent, omniscient, living, Triune, omnibenevolent Christian God.

See: the dictionary if you want a precise and detailed and reliable answer.

(Hint: assuming the following entities actually EXIST...a deity is a particular TYPE of entity. A person is also a type of entity. A ghost is also a type of entity. An angel is also a type of entity. A demon is also a type of entity. etc.)

A deity is an entity that has great power and inspires awe or reverence in "lesser" entities. Every being is an entity, but not all entities can be considered deities if there aren't any other entities (believers) around to consider it superior to themselves (divine).

Entities may be characterized not only by relationships, but also by additional properties (attributes), which include identifiers called "primary keys". Diagrams created to represent attributes as well as entities and relationships may be called entity-attribute-relationship diagrams, rather than entity–relationship models.

A deity is typically conceptualized as a supernatural or divine concept, manifesting in ideas and knowledge, in a form that combines excellence in some or all aspects, wrestling with weakness and questions in other aspects, heroic in outlook and actions, yet tied up with emotions and desires. In other cases, the deity is a principle or reality such as the idea of "soul". The Upanishads of Hinduism, for example, characterize Atman (soul, self) as deva (deity), thereby asserting that the deva and eternal supreme principle (Brahman) is part of every living creature, that this soul is spiritual and divine, and that to realize self-knowledge is to know the supreme.

A deity can be an entity, but an entity isn't necessarily a deity as an entity isn't necessarily a god and most often isn't a god.