Unfunded pensions liability - definition?

Can someone provide a definition or explanation of what an unfunded pensions liability is please?
For instance, if a company has an obligation to pay future pensions but has not made contributions (either from its own resources or collected from the pensioners) then that presumably applies as "unfunded" as the pension needs to come solely from future trading.
However suppose it has accrued amounts relating to salary of each employee but has not put these into a separate fund or bank account, is that "unfunded"? Or does it depend on the specific commitment when the pensioners retire or leave the employment?
Thanks

Duncan Philps-Tate