Calorie deficit
A calorie deficit is intake below expenditure. The familiar rule of thumb that 3,500 kcal equals a pound comes from a 1958 paper and systematically overestimates long-run loss, because expenditure itself falls as body mass falls.
Example
A 500 kcal daily deficit is 3,500 kcal a week, which the old rule renders as one pound. Over months the real figure runs lower, which is why this site shows a range rather than a date.