Refers to one's mental or emotional core, courage, or fortitude. Often used in literary or formal contexts, and in set phrases like 心胆を練る (to steel one's heart).
His fortitude is extraordinary.
Steeling one's heart, one faces the difficult situation.
Compound of 心 (heart, mind) and 胆 (gallbladder, courage). The combination suggests the seat of courage and spirit, a concept found in classical Chinese thought.