A plain-language explanation
A website is a collection of related subpages, files and functions, usually available within a single domain.
Where it helps in practice
A website is a connected set of pages, assets and functions under a domain, so one template change can affect many journeys.
The decision before implementation
Design architecture, content roles, components, publishing, maintenance and measurement before adding pages.
How to verify the result
Crawl and analysis should cover accessibility, errors, performance, indexation, security and task completion across representative paths.