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 2019 Q2 it was $172.36 billion, up by 24% year-over-year from $139.40 billion. From $456.18 billion in 2017 it increased by 27% to $578.42 billion in 2018.
Source: PayPal Quarterly Results.