n. A small fee charged to a buyer or seller by a third-party whose software or technology was used to implement the transaction.

Example Citation:
To please the Street, and to make these evanescent windfalls appear recurring, Enron had to keep expanding into new (and ever more implausible) markets — a costly proposition that required the company to assume ever more debt. And here's the rub: Enron was not merely a toll-collector, a middleman skimming the vig off every transaction. It was a "counterparty" — that is, technically, it bought and then sold all the underlying goods that were traded under its aegis.
—Stephen Metcalf, "The How and Why of Enron," New York Observer, July 1, 2002

The word vig is short for vigorish, which is bookie slang for a fee that's charged on each bet placed. (It can also mean interest paid on a loan.) It's origin seems to be the Yiddish word vyigrysh, meaning "profit or winnings."

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