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A spread in trading is the difference between the buy (offer) 
and sell (bid) prices quoted for an asset.
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Generally, the spread refers to the difference 
between two prices, rates, or yields. 

In one of the most common definitions, the spread is the gap between
the bid and the ask prices of a security or asset, like a stock, bond,
or commodity (bid-ask spread).
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Sources: 
https://www.ig.com/en/glossary-trading-terms/spread-definition
https://www.investopedia.com/terms/s/spread.asp