My definition of End-to-End Testing

End-to-end testing involves testing the complete process flow of a website or app, from start to finish. The actual process flow will depend on the type of website or app. For example, an ecommerce website end-to-end test could involve registering an account, browsing products on the site, adding products to the cart, the checkout process, connecting to payments server, the payment and subsquent confirmations received.

Wikipedia definition of End-to-End Testing

System testing, or end-to-end testing, tests a completely integrated system to verify that it meets its requirements. For example, a system test might involve testing a logon interface, then creating and editing an entry, plus sending or printing results, followed by summary processing or deletion (or archiving) of entries, then logoff.

Techopedia definition of End-to-End Testing

End-to-end testing is a methodology used to test whether the flow of an application is performing as designed from start to finish. The purpose of carrying out end-to-end tests is to identify system dependencies and to ensure that the right information is passed between various system components and systems.