A startup is a temporary organization used to search 
for a repeatable and scalable business model. — Steve Blank

Or, in the words of Dave McLure, who made it to the top-rated definition from Quora: “A startup is a company 
that is confused about what its product is, who its customers are, and how to make money. As soon as it figures out all three things, it ceases being a startup and becomes a real business. Except most times, that doesn’t happen.”

The above definitions also apply to a ‘corporate startup’. The only difference is that the latter operates in a corporate context where it might be able to leverage existing resources and assets but will additionally have to validate ‘strategy fit’ with its parent company.