Drawing from what @heather_reid and @kelly.kenyon in particular gave as their definitions, this is what mine would be:
Software testing is the endeavour of investigating a piece of software. This is in order to help your team make informed decisions about risk, quality, and success. The intention of this is ensuring that the software works as ‘well’ as it can from the perspective of both the organisation developing the software and the end-user.
Very happy to comment this as part of Lesson 3 of the Software Testing Essentials course!