If this was a traditional 401K, your "basis" in the account would have been "0".
"Basis" in a retirement plan is whatever after-tax contributions you made to that plan. When you withdraw funds from that plan, some portion of the withdrawal is a return of "basis" and that is not taxable.
Example: Assume the value of your account is $10,000 and you had made $1,000 of after-tax contributions. If you withdraw the entire amount, only $9000 is taxable income. The remaining $1000 would be a return of your basis.
If you withdrew $1000, only $900 would be taxable. The remaining $100 would be a return of your "basis".
