Refers to a pessimistic outlook or doctrine, often used in discussions of philosophy, economics, or personal attitudes.
I'm fed up with his pessimism.
Pessimism is spreading about the economic outlook.
悲観的 is a na-adjective meaning 'pessimistic', used to describe a person's attitude or a situation, while 悲観論 is the noun for the concept or doctrine of pessimism.
Compound of 悲観 (ひかん, 'pessimistic view') + 論 (ろん, 'theory, argument').