1.Definition: exclusive of anyone or anything else
"She alone believed him", "Cannot live by bread alone"
2.Definition: isolated from others
"Could be alone in a crowded room", "Was alone with her thoughts", "I want to be alone"
3.Definition: lacking companions or companionship
"He was alone when we met him", "She is alone much of the time"
4.Definition: radically distinctive and without equal
"He is alone in the field of microbiology", "This theory is altogether alone in its penetration of the problem"