Plain English Meaning
The pressure number a pilot dials into the altimeter, taken from a weather station near where they are flying, so the altimeter shows the correct height above sea level for that area.
Full Definition
The current barometric pressure value, expressed in inches of mercury (or hectopascals/millibars), provided by a nearby weather reporting station and entered into an aircraft's altimeter so the instrument reads true elevation above mean sea level at airports within that station's area. It corrects the altimeter for the actual air pressure at the surface in that location at that time.