Solitude is a state of seclusion or isolation, i.e. lack of contact with people or love. It may stem from bad relationships, deliberate choice, contagious disease, disfiguring features, repulsive personal habits, mental illness, or circumstances of employment or situation (see castaway).
Short-term solitude is often valued as a time when one may work, think or rest without being disturbed. It may be desired for the sake of privacy.
A distinction can be made between physical and mental seclusion. People may seek physical seclusion to remove distractions and make it easier to concentrate, reflect, or meditate. However, this is not necessarily an end in and of itself. Once a certain capacity to resist distractions is achieved, people become less sensitive to distractions and more capable of maintaining mindfulness and staying inwardly absorbed and concentrated. Such people, unless on a mission of helping others, don't seek any interaction with the external physical world. Their mindfulness is their world, at least ostensibly.
Still, long-term solitude is often seen as undesirable, causing loneliness or reclusion resulting from inability to establish relationships. Furthermore, it might even lead to clinical depression. However, for some people, solitude is not entirely depressing. Still others (e.g. monks) regard long-term solitude as a means of spiritual enlightenment. Indeed, marooned people have been left in solitude for years without any report of psychological symptoms afterwards.
John T. Cacioppo's, University of Chicago, 2008 book, Loneliness: Human Nature and the Need for Social Connection, outlines five distinct pathways through which social isolation contributes to increased illness and early death. He also offers an evolutionary rational for why the subjective sense of social isolation--loneliness--is so profoundly disruptive to human physiology that it impairs cognition and will power, alters DNA transcription in immune cells, and leads over time to high blood pressure.
Enforced loneliness (solitary confinement) has been a punishment method throughout history. It is often considered a form of torture. In contrast, some psychological conditions (such as schizophrenia) are strongly linked to a tendency to seek solitude.
Protective isolation is the type of isolation created in tests. This can usually be classified by the fact that one can opt out of the experiment, or the isolation. It can often be prepared for, and is generally not a negative thing. (More often than not, there is a reward for the subject's time as an experiment.)
Source isolation includes no benefits, and cannot be prepared for. Thus, it is usually undesirable, and is not very common.