The degree of air leakage or air infiltration a building has. Making a building airtight essentially means eliminating draughts. Ideally we want to have total control over how much air we're letting in to the building through designed ventilation systems, rather than cold air entering (and warm air escaping) uncontrolled through unwanted or unseen gaps.
Airtightness is typically measured in two units: air changes per hour (ACH) and air permeability (m3/hr/m2). For a typical building, there is usually little difference in the two figures. The smaller the airtightness figure the better. Under Irish building regulations new homes must have an air-tightness of 10 m3/hr/m2. The rigorous Passivhaus standard demands an airtightness of 0.6ACH or less.