The term 'zero on the Beaufort scale' refers to a measure of wind speed. The Beaufort scale is an empirical measure that relates wind speed to observed conditions at sea or on land. The scale was developed in 1805 by British Admiral Francis Beaufort and is still used today. The solution to the crossword for this term is 'CALM', which is defined as having no wind or a wind speed of zero on the Beaufort scale.