The difference between the CDS spread and par equivalent CDS spread:

PECS basis = CDS spread – PECS

It captures the divergence between the risk profile of the CDS curve and that of the bond curve. The more the PECS basis, the riskier a credit default swap (CDS) gets, and vice versa.

It is similar to the so-called standard basis, i.e., the difference between the CDS spread and Z-spread.