Someone who is lonely is unhappy because they are alone or do not have anyone they can talk to. (人) 孤独的
adjective [原级]孤单的,寂寞的;在孤单中度过的;偏僻的,人迹罕至的
A lonely situation or period of time is one in which you feel unhappy because you are alone or do not have anyone to talk to. 孤独的 (情形、时期) She felt lonely and hopeless.