Solitude, isolation, alienation, seclusion mean the state of one that is alone.
Solitude applies not only to a physical condition where there are no others of one’s kind with whom one can associate, but often to the state, physical or mental, of one who by wish or by compulsion is cut off from normal contacts (as with colleagues, neighbors, friends, or family).
Sometimes the term refers entirely to a mental state and comes very close in meaning to loneliness, implying a lack of intimate association with, rather than a separation from, others.
Isolation stresses detachment from others either because of causes beyond one’s control or because of one’s own wish. Since the term may refer to communities and to things as well as to individuals, it often suggests a cutting off physically rather than such a frame of mind as loneliness or depression.
Alienation stresses estrangement and lack or loss of adjustment either between the individual and his environment and especially his social or intellectual environment so that he is in fact isolated even when physically surrounded by multitudes of his kind or sometimes between the creator and his creation.
Seclusion implies a shutting away or a keeping apart of oneself or another so that one is either inaccessible to others or accessible only under difficult conditions. The term may connote a condition (as confinement in an asylum or prison or withdrawal from the world or from human companionship) that makes contact difficult or repels the efforts of others to establish contact.