Once' is a four-letter word that can be used to describe something that happens only one time. It can also be used to refer to a particular point in time, such as 'ONCE upon a time.' In this context, it could be used to describe the beginning of a storybook, which is the term 'Storybook Starter.' This phrase suggests the start of a narrative, usually a children's book, that will be filled with adventure and imagination.