Longman Dictionary of Contemporary English - closed

~ adj 1 shut, especially overnight or for a certain period of time  (The shops are closed on Sundays. | closed to the public/visitors etc)  (The gardens are closed to visitors in winter.) 2 restricted to a particular group of people  (closed membership)  (The golf club has closed membership.) 3 not willing to accept ideas or influences from outside  (closed society/world/way of life)  (Army officers and their families live in a very closed world. | closed mind (=someone who has a closed mind does not accept new ways of thinking or doing things)) 4 behind closed doors if official meetings or decisions take place behind closed doors, they take place secretly 5 a closed book a subject or problem that someone knows or understands nothing about  (Most of mathematics is just a closed book to me.)