Outcast, castaway, derelict, reprobate, pariah, untouchable are comparable when they mean one who has lost contact with or has been excluded from association with men in general or with a particular group.
Outcast is usually applied to a person who has been rejected by society and is forced to live without its help, its companionship, or its approval; the term need not imply a degraded or abject condition, but it does suggest a loss of the comforts that accrue from one's association with other men.
Castaway usually implies abandonment as the result of shipwreck and suggests the wretched and pitiable condition of one isolated from both human society and normal human comforts.
Derelict basically applies to property and especially to a ship abandoned and left to the mercy of the elements. In application to human beings the term emphasizes a cutting off from normal social association particularly because of irresponsible or dissolute habits and more often than outcast or castaway suggests a debased state more or less voluntarily assumed.
Reprobate basically applies to one who, because of his sins, in rejected by God. In extended use reprobate is more likely to imply the disapprobation of society than actual rejection by society, and, while it may impute grave wrongdoing, it is very likely to suggest a degree of social tolerance.
Pariah and untouchable are words basically used to denote specific socially inferior or unacceptable groups in India but in their more general applications often used without reference to the original meaning.
Pariah in such general application typically denotes a person who especially as a member of a group is, justifiably or unjustifiably, rejected or despised by society.
Untouchable is applicable not only to an individual but to a group (as a people, nation, or class) which another and supposedly superior group regards as beneath its notice or outside the sphere of its consideration.