Dew point is the temperature at which air would become saturated if cooled at constant pressure and constant moisture content, and it depends only on the actual vapour pressure — not on how warm the air happens to be now. That distinguishes it from wet-bulb temperature, which is reached by evaporative cooling and always lies between the dew point and the dry-bulb temperature; the two are equal only at 100% relative humidity. Air at 25 °C and 50% RH at sea level carries a vapour pressure of 1,581 Pa and has a dew point of 13.9 °C, against a wet-bulb of 17.9 °C.
Numerically it comes out of the saturation-pressure relation: the Magnus form Pws = 610.94·exp(17.625·T/(T + 243.04)) gives saturation pressure from temperature (agreeing with the ASHRAE Handbook of Fundamentals tables to within a few tenths of a percent over 0–60 °C), and inverting it at the air’s actual vapour pressure yields the dew point.
Its practical weight in buildings is condensation. The Glaser method of EN ISO 13788 walks the temperature and vapour-pressure profiles through a wall or roof interface by interface; wherever a layer cools below the local dew point — actual vapour pressure reaching saturation — moisture condenses inside the construction rather than on its surface, and the analysis quantifies the accumulation in g/m² per month. With a 20 °C / 50% RH interior against a −5 °C / 90% RH exterior, an interface below its dew point is exactly what flags a wall for a vapour control layer or warmer inner layers.