Entity

An entity set that does not possess sufficient attributes to form a primary key is called a weak entity set. One that does have a primary key is called a strong entity set.
For example,

The entity set transaction has attributes transaction-number, date and amount.
Different transactions on different accounts could share the same number.
These are not sufficient to form a primary key (uniquely identify a transaction).
Thus transaction is a weak entity set.
For a weak entity set to be meaningful, it must be part of a one-to-many relationship set. This relationship set should have no descriptive attributes.

The idea of strong and weak entity sets is related to the existence dependencies seen earlier.

Member of a strong entity set is a dominant entity.
Member of a weak entity set is a subordinate entity.
A weak entity set does not have a primary key, but we need a means of distinguishing among the entities.

The discriminator of a weak entity set is a set of attributes that allows this distinction to be made.

The primary key of a weak entity set is formed by taking the primary key of the strong entity set on which its existence depends (see Mapping Constraints) plus its discriminator.

To illustrate:

transaction is a weak entity. It is existence-dependent on account.
The primary key of account is account-number. transaction-number distinguishes transaction entities within the same account (and is thus the discriminator).
So the primary key for transaction would be (account-number, transaction-number).
Just Remember: The primary key of a weak entity is found by taking the primary key of the strong entity on which it is existence-dependent, plus the discriminator of the weak entity