Plain English Meaning
It's the local air pressure number you dial into your altimeter so it shows the right height above sea level. ATC or an automated weather broadcast gives you this number, and you turn a small knob to enter it.
Full Definition
The barometric pressure value, expressed in inches of mercury (or hectopascals/millibars), that a pilot sets in the altimeter's Kollsman window so the instrument reads the correct altitude above mean sea level for the local area. It is the to sea level using atmospheric assumptions.