QA
Also known as: quality assurance
QA is the discipline of establishing that software does what it should before users find out that it does not. It spans automated tests written alongside the code, exploratory testing by someone deliberately trying to break it, and acceptance testing against the agreed requirements.
It is not the same as testing at the end. Quality is largely determined by decisions made earlier — a clear specification, a testable design, tests that run on every change — and a QA phase appended to the end of a project can only measure what was built, not improve it.
The economics are not subtle. A defect found by a test as the code is written costs minutes; found in production it costs an investigation, a fix, a release, and whatever the customer experienced in between.