In general, in philosophy, entity is part of a set of interrelated terms. This following list is the result of personal experience, consider this answer just an opinion; the dialectical approach simplifies its understanding:
- An object is the counterpart of the subject in an interaction. It is any whole which is not the subject. A subject is an observeR, an object is an observeD.
- An entity is the equivalent of an object that exists by itself (from Latin ens: to exist). Notice the difference: if an object needs of a subject to exist, an entity is considered to exist by itself.
- A being is usually the same thing as an entity.
- A thing is mostly a physical object, although the definition of thing is very loose.
- A system is commonly an entity, approached from its structural perspective. Strictly, system means a set of interrelated parts; notice that such definition is recursive/fractal: any system is a set of parts and any part is a subset of a system.
- A set is a collection of objects; notice the difference with a system: while a system is a set of related parts, a set does not approach relations.
In all concepts, it is important to consider that:
a) All boundaries are subjective. A mountain can include or not the rocks 10m below its base; a wave can include or not the water drops around it; a house can include or not part of the street; an apple can include or not the water molecules that had just evaporated a nanosecond ago, a person can include or not its clothes, a rainbow can include or not the sunlight, etc. This means that perceiving a circle implies perceiving two domains: what is the circle and what is NOT the circle.
b) All objects essentially exist in our mind. Outside of us, the rainbow, the river, the rock, the sea, etc. is just a set of molecules or atoms that has no boundaries with the rest of atoms of the universe. A martian having the size of one atom would not be able to know an apple (how would he know the boundaries, which finally are human ideals?). However, some philosophical perspectives accept that objects exist as independent things, without need of the perception of an observer (see metaphysical realism).