Kaisan (開山) is a Japanese term used in reference to the founder of a school of Buddhism or the founding abbot of a Zen monastery, literally meaning "mountain founder" or "to open a mountain." Ch'an monasteries of China and Japan have traditionally been built in mountainous regions, with the name of whatever mountain it has been built upon then fixed upon the monastery as well as the founding abbot.
References
Kaisan Wikipedia(Text) CC BY-SA