What is Indefinite Imprisonment?

Legal Definition
Indefinite imprisonment or indeterminate imprisonment is the imposition of a sentence by imprisonment with no definite period of time set during sentencing. Its length is instead determined during imprisonment based on the inmate's conduct. The inmate can be returned to society or be kept in prison for life.

In theory, an indefinite prison sentence could be very short, or it could be a life sentence if no decision is made after sentencing to lift the term. In many cases, either a minimum term is imposed or the maximum that can be served is the maximum allowable by law in the jurisdiction for the particular offense.
-- Wikipedia
Legal Definition
the term used to describe being imprisoned when no time limit has been set.