Mid-market rate
The mid-market rate is the midpoint between the buy and sell prices for a currency pair on the global market, used as the reference point every other rate is measured against.
It is a reference, not an offer. No consumer provider transacts at it, because moving money across a border has real cost.
Its usefulness is as a yardstick: quoting your rate against the mid-market rate is how a provider makes its margin legible, and refusing to is how one hides it.
Rates move continuously. A rate shown on a marketing page is indicative; the rate that applies to your transfer is the one confirmed on the review screen before you press send.