Carthusian
Concise Oxford English Dictionary © 2008 Oxford University Press:
Carthusian /kɑːˈθjuːzɪən/
▶noun a monk or nun of an austere contemplative order founded by St Bruno in 1084. ▶adjective relating to the Carthusians.
– origin from med. L. Carthusianus, from Cart(h)usia, L. name of Chartreuse, near Grenoble, where the order was founded.
'Carthusian' also found in these Oxford entries:

