PayPal defines this as:
Total Payment Volume or “TPV” is the value of payments, net of payment reversals, successfully completed through our Payments Platform, excluding transactions processed through our gateway products.
Last reported quarter 2018 Q3 it was $143.00 billion, up by 24% year-over-year from $115.22 billion. From $355.38 billion in 2016 it increased by 28% to $456.18 billion in 2017.
Source: PayPal Quarterly Results.