(Latin, from per-, "through", and ager, "field"; literally, "a person who moves through the fields", from which the actual meaning "traveller, foreigner", and later "pilgrim" is derived).
In Roman republic and empire, the term peregrinus was used to signify any free individual who was not a Roman citizen, and who therefore did not enjoy the protections normally applicable to Roman citizens under Roman law.
In 212, the term lost significance, when Emperor Caracalla published the Constitutio Antoniniana, guaranteeing all inhabitants of the empire full benefit of Roman citizenship, and thus, equal rights under the law.
With the rising popularity of Christian pilgrimages during the middle ages, the term, so evocative of travel, came to mean "pilgrim", and sometimes "Crusader" (bear in mind that Crusaders were, technically speaking, armed pilgrims).