资料来源 : Free On-Line Dictionary of Computing

partial equivalence relation
     
        (PER) A relation R on a set S where R is symmetric (x R y => y
        R x) and transitive (x R y R z => x R z) and where there may
        exist elements in S for which the relation is not defined.  A
        PER is an equivalence relation on the subset for which it is
        defined, i.e. it is also reflexive (x R x).