An entity whose existence depends on the existence of another entity is called a weak (or dependent) entity. For eg: the dependent of an employee is a weak entity, whose existence depends on the entity employee.
An entity is an object that exists and is distinguishable from other objects. For instance, John Harris with S.I.N. 890-12-3456 is an entity, as he can be uniquely identified as one particular person in the universe.
the entity is called weak entity if it doesn't have its on primary key,it depends on the strong entity. ex:-if loan is strong entity set then payment is the weak entity which depends on the strong entity loan