Overround (margin)
The overround is the bookmaker's built-in margin: the amount by which the implied probabilities of all outcomes exceed 100%.
Take one bookmaker's match-result prices of 2.10 on the home side, 3.40 on the draw and 3.60 on the away side. Their implied probabilities are 47.62%, 29.41% and 27.78%, which add up to 104.81%. The overround is the 4.81% above 100, and it is there whichever way the match ends.
It reads just as well as stakes. To collect 100 units whatever the result, a punter would stake 47.62 on the home side, 29.41 on the draw and 27.78 on the away side, 104.81 units in all. The 4.81 units that never come back are the margin, spread across the market.
Two-way markets show it plainly. At 1.90 on both sides the book sums to 105.26%, an overround of 5.26%. A tighter overround means more of the money staked across a market comes back to punters, and it only compares fairly like for like, the same market on the same event at two bookmakers.
In the OddsRelay feed
Overround belongs to one bookmaker on one market. On OddsRelay's raw feed a tool takes one bookmaker's entry in bookmakers[], picks a market by key, and adds up one divided by each outcomes[].price. The sum only means something when that bookmaker prices every outcome of the market, so a market with an outcome missing is left out.
A matched feed is the wrong input for it. Each outcome's back[] holds prices from different bookmakers, best price first, so taking the first entry of each outcome mixes several books and describes no single bookmaker's margin. Exchange prices, carried on raw as back_price and lay_price with no commission applied, tend to sum close to 100% on the back side, since an exchange earns its commission on winnings.
Because raw lists every bookmaker quoting an event, a tool can set each bookmaker's overround on the same market key side by side and follow it over time. Each bookmaker's markets[].last_update shows when that market last changed, so a shift in one book's margin can be dated to the change that caused it.
Try it on your own key
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