A claim secured by a lien in the debtor's property by reason of the debtor's agreement or an involuntary lien such as a judgment or tax lien. The creditor's claim may be divided into a secured claim, to the extent of the value of the collateral, and an unsecured claim equal to the remainder of the total debt. Generally a secured claim must be perfected under applicable state law to be treated as a secured claim in the bankruptcy. The role of the trustee is different under the different chapters. More on trustees.
Most consumer debts are unsecured.
Further definitions are found in Section 101 of the Bankruptcy Code.