Define weak and strong entities:
An entity type that has an existence dependency is called weak entity. Thus, an
instance of a weak entity cannot exist independently but depends on the
existence of another entity (strong entity) instance. The weak entity is also
referred to as child, or dependent or subordinate. In E-R diagram, the existence
dependency of weak entity is indicated by double line rectangle.
An entity type that has no existence dependency is called strong entity. It can
exist without depending upon the existence of another entity. The strong entity
is also referred to as parent, or owner, or dominant.