alka, in Baltic religion, an open-air religious site, a natural sanctuary—forest, hill, river—that was sacred and inviolate. Trees could not be cut in such forests, sacred fields could not be plowed, and fishing was not allowed in the holy waters. The rituals of various religious cults, involving animal sacrifice and human cremation, took place at the alkas. The sense of the ancient alka is preserved in the modern Lithuanian word alkvietė, meaning any holy place or site of worship.