The Vig, short for Vigorish, is defined by Webster's dictionary as the following:
"a charge taken (as by a bookie or a gambling house) on bets; also :
the degree of such a charge <a vigorish of five percent>"In layman's terms, The Vig is what gives the casino an advantage over you. If it weren't for the Vig, casinos would not be able to stay in business and we wouldn't have the famous strip in Las Vegas.
A simple example can be explained using a coin toss. With a coin toss, the chances of tossing Heads is exactly the same as tossing Tails. You have a 50% chance of either one. Therefore, there is no advantage to playing one side or the other. However, if a casino offered this game, they may charge you $1.10 to play, but only return $1.00 if guessed correctly. Therefore, if you guess correctly, you win $1.00. If you guess incorrectly, you lose $1.10. This would give the casino and edge, or Vig, of 4.5%. To calculate this, you would use this formula:
((percent chance of losing * loss ) - (percent chance of winning * win)) / (amount wagered) =
((.50 * $1.10) - (.50 * $1.00)) / ($1.10) = .045 or
the house with have a 4.5% Vig on this bet.
You should note that in this example, you are wagering $1.10 to win $1.00. The same Vig would apply to wagering $1.00 to win $0.91.
www.semibluff.com/Vegas/thevig/odds.htmETA: link