The name Kai is primarily a gender-neutral name of Hawaiian origin that means Ocean.
Kai is a name with various unrelated origins. Not only is the name Hawaiian, but it is a diminutive name found in many European languages including German, Swedish, Norwegian, Danish, Finnish, Dutch, Frisian, and Estonian. It can derive from the names Gaius, Gerhard, and various other names. For the feminine usage it can be a form of Kaia, which derives from the name Katarina, or the Estonian name meaning pier or wharf.
The name is also found in Japanese with multiple possible meanings including ocean, shell, open, restoration, or recovery.