What is a comic book's 'key issue'?

What is a comic book's 'key issue'?

Mark Zaid (Owner of Esquire Comics) gives expert video advice on: What are the different types of comic book covers? and more...

A key Issue of a comic book refers to something extraordinary or unique that has happened in that particular comic book. For example it could be, in Action Comic Books, when Supergirl makes her first appearance 15 or so years after Superman and Action Comics has actually begun.