What is the difference between absolute and gauge pressure?

Gauge pressure is measured relative to atmospheric pressure (reads zero at atmosphere), while absolute pressure is measured relative to perfect vacuum (reads atmospheric pressure at sea level ~14.7 psia or 101.3 kPa). The relationship is: Absolute pressure = Gauge pressure + Atmospheric pressure. Absolute pressure measurement is essential for vacuum systems, gas laws calculations, and altitude-sensitive applications where atmospheric pressure varies.