开心 (kāi xīn) literally means to open the heart and is a common, informal way of saying happy or joyful. It is interchangeable with 高兴 (gāo xìng) in many contexts but feels slightly warmer and more heartfelt. 开心 is widely used in modern spoken Mandarin and social media. It can be used predicatively (我很开心, I am happy) or attributively (开心的笑容, a happy smile).