a hypothetical body of infinitely large mass capable of absorbing or rejecting unlimited quantities of heat without undergoing appreciable changes in temperature, pressure, or density.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
Examples from the Web for heat reservoir
Historical Examples of heat reservoir
He had to pump that heat into a heat-reservoir—a water tank probably—to protect himself.The Ultimate Weapon
John Wood Campbell