Agree what done looks like.
Behaviour-driven development, introduced by Dan North, writes acceptance criteria as concrete examples in plain language.
Scenario: Customer downloads an invoice
Given I'm logged in as a business customer
And I have an invoice for August
When I choose “Download PDF”
Then the August invoice downloads as a PDF
And it shows our VAT numberExamples over rules
Concrete cases catch what abstract rules miss.
Written together
Product Owner, developer and tester: the three amigos.
Becomes the test
The same words can drive automated tests.
Source: Dan North & Associates: Introducing BDD ↗
Examples first. Written together. Tested for real.