What "concept" means in the definition of an entity

One definition of "entity" in the Wikipedia page for entity-relationship model as follows: > An entity is a thing that exists either physically or logically. An entity may be a physical object such as a house or a car (they exist physically), an event such as a house sale or a car service, or a concept such as a customer transaction or order (they exist logically — as a concept). I don't really understand what concept means in this definition and why a transaction or order is listed as a concept. A customer transaction or an order exists on paper (as bills) so can I say they exist physically? Besides, the event is also not a physical object so it should be a concept?