28-day strength
The conventional age at which concrete strength is measured for acceptance. It is a practical convention, not the point at which concrete stops gaining strength.
What does 28-day strength mean?
The 28-day mark was chosen historically as a reasonable compromise between construction schedules and the strength gain curve of ordinary portland cement. Mixes containing fly ash or slag continue gaining substantially past this age, so testing them at 28 days understates their eventual performance, and some specifications allow later acceptance ages. Concrete typically reaches a large fraction of its 28-day strength within the first week.
In practice
Waiting for 28-day breaks is often the last item holding up closeout paperwork on a concrete package.