Definitions for "Sunk Cost"
In economics, a sunk cost is a cost that has already been incurred, and therefore cannot be avoided by any strategy going forward.
Any expenditure that has already taken place and can not be undone.  Decisions should not be made based on sunk costs.
A cost that has already been incurred and cannot be reversed, which therefore cannot be avoided by current or future action. Sunk costs should therefore be irrelevant to current decisions.
an investment which is unrecoverable
Money a company pays that relates to an employee's employment, include lights etc.