Notional Value
The total value of a leveraged position's assets. This term is commonly used in the options, futures and currency markets because in them a very little amount of invested money can control a large position (have a large consequence for the trader).
Investopedia Commentary
For example, one S&P 500 Index futures contract obligates the buyer to 250 units of the S&P 500 Index. If the index is trading at $1,000, then the single futures contract is similar to investing $250,000 (250 x $1,000). Therefore, $250,000 is the notional value underlying the futures contract.
Related Links
Futures Fundamentals
Options Basics Tutorial
A Primer On The Forex Market
See also: Derivative, Futures, Leverage, Notional Principal Amount, Open Interest