Basis

The amount you contributed to your Individual Retirement Arrangement (IRA) on which you already paid tax. If you filed Form 8606 in a prior year, you can get the basis from the prior year's form. If this is the first time you have had to prepare Form 8606, your basis will be the total nondeductible contributions to your IRA. (from MSN.com)

Also see 'adjusted basis'

See also: Adjusted Basis, Individual Retirement Arrangement (IRA)