INVEST criteria
INVEST is a mnemonic, coined by Bill Wake in 2003, for the qualities of a good user story: Independent (can be worked on without depending on other stories), Negotiable (not a rigid contract, room to discuss the implementation), Valuable (delivers value to the user or business), Estimable (the team can size it), Small (fits in a single sprint), Testable (acceptance criteria can be verified).
INVEST is the most-cited checklist in agile story-writing because the qualities are operationally testable: in refinement, the team can walk through the list for each story and reject ones that fail. The hardest in practice is Independent, stories often have technical dependencies that don't show up in the user-facing description. The most-violated is Small, teams routinely commit stories that turn out to be epics. INVEST pairs naturally with story splitting (a too-big story becomes multiple INVEST-compliant stories) and with definition of ready (stories must be INVEST-compliant to enter a sprint).