Practical completion is the contractual milestone at which the works are complete enough for the employer to take and use them, even if minor snags remain. Its exact meaning depends on the contract and, where undefined, on case-by-case judgement.
Its consequences are significant: liquidated damages stop, half the retention is usually released, insurance and possession pass, and the defects liability period begins.
Because so much turns on the date, PC is often argued. Records of the state of the works at the time, inspections, outstanding items and beneficial use are what settle those arguments.