Pricing and rates
Mid-market rate
Midpoint between the best bid and the best ask. A reference, never an executable price.
Mid-market serves as a common comparison point because it favours neither buyer nor seller. Nobody trades at exactly that price: every real execution happens on one side of the book or the other.
A mid-market rate only means something with two pieces of information: its source and its timestamp. Announcing "the market rate" from a reference shifted by 0.3% is worth 300 currency units on 100,000, invisible to the naked eye. It is the first place a cost can hide.