刚毅 is a formal, literary adjective describing a person's character — someone who is resolute, unyielding, and steadfast in the face of difficulty. It combines 刚 (firm, hard) and 毅 (determined, persevering), and is often used in written Chinese or formal speech to praise someone's moral strength or courage. It's stronger than 坚强 and carries a sense of dignified toughness.