Define:

(a) Indicated Altitude;

Is the altitude shown on the face of the instrument when set for the atmospheric pressure actually existing at the time and place of flight.

(b) Calibrated Altitude;

Is the indicated altitude corrected for Instrument Error, Position Error and Lag.

(c) True Altitude;

Is the pressure altitude corrected for temperature. 

(d) Pressure Altitude (PA);

Is the calibrated altitude with the barometric scale set to the standard sea level pressure of 1013.2 hPa. 

It is used on the various air navigation computers in determining the true altitude and in determining the True airspeed.

Many performance graphs are related to pressure altitude.

(e) Density Altitude (DA);

Is the altitude in standard atmosphere corresponding to the air density at a particular place and set of conditions. 

It is the pressure altitude corrected for temperature: the higher the temperature, the higher the density altitude. 

It is used in determining engine performance, and also the performance of the aircraft itself.

Often used for performance graphs.