Mean sea level. When we say 5000' MSL we mean 5000 feet above sea level. I know the abbreviation doesn't make any sense in the way we use it, but that's its colloquial use anyway.
Mean Sea Level (datum for altitude measurement)
mean sea level. A tidal datum. The arithmetic mean of hourly heights observed over the National Tidal Datum Epoch. Shorter series are specified in the name; e.g., monthly mean sea level and yearly mean sea level.