Definition of Existence Dependency
Whenever the existence of an entity depends on the existence of another entity, that relationship is described as an existence dependency (ED).
For example, suppose there are two entities named Author and Book. A writer might have been provided with an advance to write a book for a publisher, but if this is the first book written by the writer for this publisher, there will be no entry in the Book entity until the contracted book has been written and published. In this case, the relationship between Author and Book is 1:0, indicating that the existence of an instance of Book is optional.