Cost Basis
Definition of 'Cost Basis'
1. The original value of an asset for tax purposes (usually the purchase price), adjusted for stock splits, dividends and return of capital distributions. This value is used to determine the capital gain, which is equal to the difference between the asset's cost basis and the current market value. Also known as "tax basis".
2. The difference between the cash price and the futures price of a given commodity. 

Investopedia explains 'Cost Basis'
1. Using the correct tax basis is important especially if you reinvested dividends and capital gains distributions instead of taking the earnings in cash. Reinvesting distributions increases the tax basis of your investment, which you must account for in order to report a lower capital gain (and therefore pay less tax). If you don't use the higher tax basis, you could end up paying taxes twice on the reinvested distributions.
For example, say you bought 100 shares of a stock for $1,000 last year and you reinvested the $100 of dividends distributed from the company. The next year, you received $200 in dividends and capitalgains distributions, which you again reinvested. Since tax law considers these reinvested earnings as paid to you even though you didn't actually have the cash in hand, your adjusted cost basis when the stock is sold should be recorded at $1,300 instead of the original purchase price of $1,000. Thus, if the sale price is $1,500, the taxable gain would only be $200 ($1,500  $1,300) instead of $500 ($1,500  $1,000). If you record the cost basis as $1,000, you'll end up paying more taxes than you have to. 2. For example, if particular corn futures contract happens to be trading at $3.50, while the current market price of the commodity today is $3.10, there is said to be a $0.40 basis. 
