Residual volume is the air that remains in the lungs even after the most forceful possible exhale, and can't be measured by spirometry alone since it can never be breathed out.
Even after a maximal exhale, residual volume keeps the alveoli from collapsing.
Spirometry only measures air that moves in and out. RV can never be exhaled, so it requires body plethysmography, helium dilution, or nitrogen washout to detect the trapped air.
Roughly 1.2 L in an average adult. It rises with air trapping in obstructive disease like emphysema.
Air trapping — the lungs can't fully empty, as in COPD and emphysema, so more air stays behind after each breath.
RV is the air left after a maximal exhale; FRC is the larger volume left after a normal, relaxed exhale. RV is part of FRC.