What is the difference between calorie deficit and TDEE?

Total Daily Energy Expenditure (TDEE) is the estimated number of calories your body uses each day through basic bodily functions (breathing, digestion, etc.) and physical activity. 

A calorie deficit happens when you consume fewer calories than your TDEE. In theory, this gap leads your body to use stored energy to make up the difference. 

Both of these concepts are approximations. TDEE calculations are based on population averages and don’t capture individual factors or shifts in metabolism, activity, and health. Because of this, calorie deficits and TDEE should be used as general guides.