A weak entity is an entity that cannot be uniquely identified by it's own attributes alone. It instead relies on a relationship with another entity (known as it's owner entity), to uniquely identify it.

Weak entities must participate in a one-to-many relationship with it's owner entity, called it's identifying relationship (typically a belongs to relationship). Weak entities must have total participation in their identifying relationships sets meaning every weak entity must be related to an owner.

The identifying key of a weak entity is a combination of it's own partial key (discriminator) and the primary key of the owner entity. In a entity-relationship diagram the identifying key is underlined with a dashed line, and weak entities are drawn with thick lines and the same with their identifying relationship.