An obligation, debt, or responsibility owed to someone.
Long-term liabilities are debts that become due, or mature, at a date that is more than a year into the future. An example of this is a student loan. Let's say John, a freshman in college, obtains a student loan for 25,000 and the bank does not require loan payments until 6 months after he graduates, i.e. 4.5 years after the loan was originated. This is an example of a long-term liability.
"Notes Payable" and "Bonds Payable" are also examples of long-term liabilities, and they often introduce an interesting distinction between current liabilities and long-term liabilities presented on a classified balance sheet.
Let's say Company X obtains a 100,000 Note Payable that requires 5 annual payments of 20,000 starting 1/1/14. On Company X's 12/31/12 balance sheet, a long-term liability for 100,000 would be reported, but what about the balance sheet as of 12/31/13? Since Company X is required to make a 20,000 payment on 1/1/14, which is less than one year away, a current liability of 20,000 and a long-term liability of 80,000 would be reported on its balance sheet as of 12/31/13.
Continuing one year forward, Company X would report a current liability of 20,000 and a long-term liability of 60,000 on its balance sheet as of 12/31/2014.
What this example presents is the distinction between current liabilities and long-term liabilities. Despite a Note Payable, Bonds Payable, etc., starting out as a long-term liability, the portion of that debt that is due within a year has to be backed out of the long-term liability and reported as a current liability.
See below for the balance sheet reporting treatment of the current and long-term liability portions of the Note Payable from initiation to final payment.