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Dew point is a fundamental concept in meteorology that refers specifically to the temperature at which air becomes saturated with moisture, leading to the beginning of condensation. When the air temperature cools to the dew point, the air cannot hold all the moisture it contains, and water vapor begins to condense into liquid water, which can lead to the formation of dew, fog, or clouds. This measurement is crucial for understanding humidity and predicting weather conditions, as higher dew points typically indicate more humid air.

The other choices do not accurately reflect the definition of dew point. The temperature at which air is completely still does not correlate to any meteorological term but can pertain to stillness of weather patterns unrelated to moisture content. The point where rain begins to fall relates more to precipitation processes rather than the temperature measurement of dew point. Lastly, while humidity levels are relevant, the temperature of the highest humidity level is not a recognized term in meteorology pertaining to dew point. Thus, option B is the correct and precise definition.