Implied volatility is a parameter part of an option pricing model, such as the BlackScholes model, which gives the market price of an option. The implied volatility shows how the marketplace views where volatility should be in the future. Since implied volatility is forwardlooking, it helps to gauge the sentiment about the volatility of a stock or the market. However, it does not forecast the direction in which an option will be headed.
Implied volatility is not directly observable, so it needs to be solved using the five other inputs of the model: the market price of the option, the underlying stock price, the strike price, the time to expiration and the riskfree interest rate. The implied volatility is calculated by taking the market price of the option, entering it into the BS formula and back solving for the value of the volatility.
There are various approaches to calculating the implied volatility. One simple approach is to use an iterative search, or trial and error, to find the value of an implied volatility. Suppose that the value of an atthemoney call option Walgreens Boots Alliance, Inc. (WBA) is $3.23 when the stock price is $83.11, strike price is $80, riskfree rate is 0.25% and the time to expiration is one day. The implied volatility can be calculated using the BS model, given the parameters above, by entering different values of implied volatility into the option pricing model. For example, start by trying an implied volatility of 0.3. This gives the value of the call option of $3.14, which is too low. Since call options are an increasing function, the volatility needs to be higher. Next, try 0.6 for the volatility; that gives a value of $3.37 for the call option, which is too high. Trying 0.45 for the implied volatility yields $3.20 for the price of the option, so the implied volatility is between 0.45 and 0.6. This procedure can be done multiple times to calculate the implied volatility. In this example, the implied volatility is 0.541 or 54.1%.
Historical volatility, unlike implied volatility, is realized volatility over a given period and looks back at past movements in price. One way to use implied volatility is to compare it with historical volatility. From the example above, if the volatility in WBA is 23.6% on Nov. 30, 2017, looking back over the past 30 days the historical volatility is calculated to be 23.5%, giving it a moderate level of volatility. Comparing this to the current implied volatility, it should alert a trader that there may or may not be an event that can affect the stock price.

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Implied Volatility  IV
The estimated volatility of a security's price. 
Volatility
1. A statistical measure of the dispersion of returns for a given ... 
Historical Volatility  HV
The realized volatility of a financial instrument over a given ... 
Volatility Swap
A forward contract whose underlying is the volatility of a given ... 
VIX  CBOE Volatility Index
The ticker symbol for the Chicago Board Options Exchange (CBOE) ... 
Implied Contract Terms
Items that a court will assume are intended to be included in ...