The energy efficiency of a process is defined to as:
where W is the amount of mechanically useful work done by the process (in joules), and Energy is the quantity of energy (again, in joules) used to run the process.
Energy efficiency can also refer to the use of techniques or practices that minimize the use of energy.
An efficiency of 1 (100%) is impossible - the second law of thermodynamics prohibits it. Theoretically, the mutual annihlation of matter and antimatter would be completely efficient (but the collection of the released energy would not be)