An entity is a thing in the physical world. It can relate both to physical objects or more abstract concepts that have a spatial presence (e.g. administrative areas are not physical objects as such, but they have physical properties that define their location and extend). An entity can only exist once.
A Feature is an abstraction of a specific entity in the map. It exists only in its digital form.
Feature Class is a synonym for "Feature Type". The preferred term is "Feature Type".
A Feature Type is a type of entities with common properties as described in the Overture Schema.
Instance is a synonym for "Feature". The preferred term is "Feature".
Object is a synonym for "Instance" or "Feature". The prefered term is "Feature".