Compressor map

Pressure ratio (y-axis)

The absolute inlet pressure divided by the absolute outlet pressure.

Mass air flow rate (x-axis)

The mass of air flowing through the intake/compressor over time. Usually lbs/min or CFM.

Surge line

The left hand boundary of the map, where the air flow is too low for the current pressure ratio. Blow off valves or Bypass valves are often used to relieve excess pressure to prevent sudden off throttle induced pressure surges.

Choke line

The right hand boundary of the map, where the turbo is approaching its flow limit

Overspeed line

The top boundary of the map, where the turbo is approaching its maximum shaft speed. Past this point, the turbo is liable to "overspin" and cause catastrophic bearing failure.

Turbo speed lines

The lines of constant turbo shaft speed.

Efficiency islands

The concentric regions on the map representing compressor efficiency. The smallest island in the center of the map is the "sweet spot", or where turbo is the most efficient.

An excellent source of more information can be found here.

Pressure Ratio

Expressed as $\frac{P_{outlet}}{P_{inlet}}$ or, more commonly, $\frac{P_{MAP}-P_{ICdrop}}{P_{ambient}-P_{id}}$

Where $P_{MAP}$ is manifold absolute pressure, $P_{ambient}$ is ambient air pressure.

Note that $P_{ICdrop}$ is intercooler pressure drop (pressure difference between $P_{outlet}$ and MAP), and $P_{id}$ is intake pressure drop (pressure difference between $P_{inlet}$ and $P_{ambient}$).

Alternately, if boost is known, it is approximated by $\frac{P_{ambient}+P_{boost}}{P_{ambient}-P_{id}}$. Intercooler drop is ignored here for simplicity.