Basal Metabolic Rate - the caloric burn of just being alive.
Basal Metabolic Rate (BMR) is the amount of calories one burns in a typical day if they did essentially nothing but stay in bed. It is the rate of calories your system spends to maintain itself doing things like growing, repairing, pumping blood, operating your immune system, processing blood, and operating your brain.
Your liver takes up the highest amount of energy in your BMR, followed by your brain, then all your muscles, then the other organs.
BMR can shift over time in response to environmental factors or changes in health.