Abbreviation is a function used by OneLoop and PaVeReduce for generating smaller files when saving results to the hard disk. The convention is that a definition like GP = GluonPropagator should be accompanied by the definition Abbreviation[GluonPropagator] = HoldForm[GP].
Overview, $Abbreviations, OneLoop, PaVeReduce, WriteOut, WriteOutPaVe, GluonPropagator, GluonVertex, QuarkPropagator.
GP[p, {\[Mu], a}, {\[Nu], b}]\Pi _{ab}^{\mu \nu }(p)