Plain English Meaning
It is the agreed-upon pressure number — 29.92 — that pilots dial into the altimeter at high altitudes so everyone is measuring height from the same starting point.
Full Definition
A fixed altimeter setting of 29.92 inches of mercury (or 1013.2 hectopascals) used by all aircraft when operating at or above the flight levels, so that every aircraft references the same pressure datum regardless of the actual local barometric pressure.
Context Anchor
Seen in altitude, pressure altitude, and instrument flight discussions, especially when aircraft operate at higher assigned altitudes using a common reference.