The word 'Peregrine' comes from a Latin term meaning 'alien'
or 'foreigner' (pereger = beyond the borders, ager = land, i.e., 'beyond
one's own land'). A planet is defined as peregrine when it has no level of
rulership over its position. That is, it is not placed in the sign(s) that it
rules, nor those where it is exalted, nor does it rule the triplicity, or the
terms or face where it is located.
NB: some recent authors wrongly suggest that a planet cannot be peregrine if it is in a sign of debility. This is incorrect - for more on this see 'The Definition of Peregrine' by Deborah Houlding