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Arbitrage is the strategy of instantly trading an asset (Example: Bitcoin) in different markets (exchanges) to profit from small differences in the asset's listed price. Traders can move between different exchanges looking for small differences in the prices of different assets.

There are price differences between exchanges

Observe that the BTC/USDT pair is trading on Binance at $26578.45, while the same BTC/USDT is trading on Bitfinex at $26575.46. That's a $2.99 ​​difference. A trader can buy BTC on Bitfinex and sell on Binance for a profit of $2.99 ​​dollars per BTC.