In Polish, it is a feminine given name that is a diminutive form of Karolina, a derivative of Karl .
[1] Kája is a Czech unisex given name that is a diminutive form of Karolína, Karla and Karel, also derivatives of Karl.
[2] In Slovene, Kaja is often a diminutive form of Kajetana, Karla and Katarina, names deriving from Caietanus (Cajetan), Karl and Aikaterine, respectively.
According to one interpretation, the name is derived from the Greek word hekateros, meaning "each of both".
According to another theory, the name has its origin in the Greek word katharos (purity).