The equivalent to 100,000 units of the quote currency in a forex trade. A standard lot is similar to trade size. It is one of the three commonly known lot sizes; the other two are mini-lot and micro-lot. 

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A standard lot represents 100,000 units of any currency, whereas a mini-lot represents 10,000 and a micro-lot represents 1,000 units of any currency. A one-pip movement for a standard lot corresponds with a $10 change. For example, if you wish to buy 100,000 U.S. dollars, then you are buying a standard lot with 100,000 units as your trade size.