Learner's definition of PRISONER
a person who is kept in a prison
a person who has been captured and is being kept somewhere
He was captured by rebel forces and kept as their prisoner for several months before they set him free.
They took him prisoner. = He was taken prisoner. [=they captured him and made him their prisoner]
The families were held/kept prisoner for four days.
— sometimes used figuratively
take no prisoners
to deal with the people you are competing with or trying to defeat in a very harsh way without mercy
see also take-no-prisoners