开心 (kāixīn)

开心 describes a feeling of happiness and delight, often in a warm, personal context.

Meanings

今天过得很开心，谢谢你们！ Jīntiān guò de hěn kāixīn, xièxiè nǐmen! Today was really fun, thank you all!

看到孩子们笑，我就开心了。 Kàn dào háizimen xiào, wǒ jiù kāixīn le. Seeing the children smile makes me happy.

希望你每天都开心快乐。 Xīwàng nǐ měitiān dōu kāixīn kuàilè. I hope you are happy and joyful every day.

他一听到好消息就特别开心。 Tā yī tīng dào hǎo xiāoxi jiù tèbié kāixīn. He became especially happy as soon as he heard the good news.

开 means "to open" and 心 means "heart." When your heart opens up, joy pours in.