is the altitude above the surface, also know as, AGL. Above Ground Level.
Absolute altitude
is the actual distance above mean sea level, or, MSL.
True altitude
altitude corrected for non-standard day pressure. It is the altitude that is indicated when the altimeter is set to standard pressure (29.92 "Hg). Needed for most performance tables/charts.
Pressure altitude
altitude corrected for non-standard day pressure AND non- standard day temperature.
Density altitude
altitude read directly from the altimeter (uncorrected) when it is set to the current altimeter setting.
Indicated altitude