Maker fee / maker rebate
What a venue charges (or pays) when your resting limit order is filled by someone else — $0 on Polymarket, $0 on most Kalshi series.
Posting a limit order that rests in the book makes you a maker. Polymarket charges makers nothing and redistributes taker fees to them as rebates; Kalshi charges no maker fee on most series and a reduced rate on a few. Arbitrage legs are almost always taker orders, because the opportunity is gone before a resting order would fill.
Related terms: Taker fee · Order book depth