What are 'Greeks'
"Greeks" is a term used in the options market to describe the different dimensions of risk involved in taking a position. These variables are called Greeks because they are typically associated with Greek symbols. Each risk variable is a result of an imperfect assumption or relationship of the option with another underlying variable. Traders use different Greek values, such as delta, theta, and others, to assess options risk and manage option portfolios.
Breaking Down the 'Greeks'
Greeks encompass many variables. These include delta, theta, gamma, vega, and rho, among others. Each one of these variables/Greeks has a number associated with it, and that number tells traders something about how the option moves or the risk associated with that option.
The number or value associated with a Greek changes over time. Therefore, sophisticated options traders may calculate these values daily to assess any changes which may affect their positions or outlook, or to check if their portfolio needs to be rebalanced.
Here are several of the main Greeks traders look at.
Delta
Delta represents the rate of change between the option's price and a $1 change in the underlying asset's price. In other words, the price sensitivity of the option relative to the underlying. Delta of a call option has a range between zero and one, while the delta of a put option has a range between zero and negative one. For example, assume an investor is long a call option with a delta of 0.50. Therefore, if the underlying stock increases by $1, the option's price would theoretically increase by 50 cents.
Theta
Theta represents the rate of change between the option price and time, or time sensitivity. Theta indicates the amount an option's price would decrease as the time to expiration decreases. For example, assume an investor is long an option with a theta of 0.50. The option's price would decrease by 50 cents every day that passes, all else being equal. If three trading days pass, the option's value would theoretically decrease by $1.50.
Gamma
Gamma represents the rate of change between an option's delta and the underlying asset's price. This is called secondorder price sensitivity. Gamma indicates the amount the delta would change given a $1 move in the underlying security. For example, assume an investor is long one call option on hypothetical stock XYZ. The call option has a delta of 0.50 and a gamma of 0.10. Therefore, if stock XYZ increases or decreases by $1, the call option's delta would increase or decrease by 0.10.
Vega
Vega represents the rate of change between an option's value and the underlying asset's implied volatility. This is the option's sensitivity to volatility. Vega indicates the amount an option's price changes given a 1% change in implied volatility. For example, an option with a Vega of 0.10 indicates the option's value is expected to change by 10 cents if the implied volatility changes by 1%.
Rho
Rho represents the rate of change between an option's value and a 1% change in the interest rate. This measures sensitivity to the interest rate. For example, assume a call option has a rho of 0.05 and a price of $1.25. If interest rates rise by 1%, the value of the call option would increase to $1.30, all else being equal. The opposite is true for put options.
Other Greeks
Some other Greeks, with aren't discussed as often, are lambda, epsilon, vomma, vera, speed, zomma, color, ultima.

Vega
Vega is a measurement of an option's sensitivity to changes in ... 
Gamma Neutral
Gamma neutral hedging is an option risk management technique ... 
Gamma
Gamma is the rate of change for delta with respect to an option's ... 
Gamma Hedging
Gamma hedging is an options hedging strategy designed to reduce, ... 
Speed
The rate at which the gamma of an option or warrant will change ... 
Delta Neutral
Delta neutral is a portfolio strategy consisting of positions ...

Trading
Using the "Greeks" to Understand Options
The Greeks provide a way to measure the sensitivity of an option's price to quantifiable factors. 
Trading
Option Greeks: The 4 Factors to Measuring Risks
In this article, we'll look at Greek risk measures: delta, theta, vega, gamma and explain their importance that will help you better understand the Greeks. 
Trading
Options Greeks
Get to know the essential risk measures and profit/loss guideposts in options strategies. 
Trading
Options Trading Strategies: Understanding Position Delta
Learn more about the position delta hedge ratio and how it can tell you the number of contracts needed to hedge a position in the underlying asset. 
Trading
How and Why Interest Rates Affect Options
The Fed is expected to change interest rates soon. We explain how a change in interest rates impacts option valuations. 
Trading
Using Options Tools To Trade ForeignExchange Spot
Find out how delta, gamma, risk reversals and volatility can all help predict movements in the cash market.

What are common delta hedging strategies?
Learn about common delta hedging strategies, including how to make a position in options delta neutral by offsetting risk ... Read Answer >> 
What does it mean to say that a straddle is "delta neutral?"
Learn what the option Greek delta is and what makes a deltaneutral position, and see an example illustrating a deltaneutral ... Read Answer >> 
How can I calculate the delta adjusted notional value?
Learn how to calculate the delta adjusted notional value of an options contract and why gross notional value cannot be used, ... Read Answer >> 
What does high open interest tell you about an option?
Learn about the open interest of options contracts and what a high and a low open interest indicate about the liquidity of ... Read Answer >> 
What is index option trading and how does it work?
Learn about stock index options, including differences between single stock options and index options, and understand different ... Read Answer >>