Definition - What does Plenary Power mean?
Plenary power is an absolute power that is complete and allows for the holder to make unilateral decisions regarding a certain subject.
In the U.S. legal system, there are certain times where the three different branches of government could have plenary power, such as absolute power over commerce or immigration; but, more often than not, the power is limited in time.
Justipedia explains Plenary Power
Companies and individuals can give plenary power in the same way that power of attorney is given.
Judges also have plenary power over individual cases that come before them, in that they are allowed to determine the specific application of the law in terms of what sentence is meted out.