There are various words and phrases that can be used as synonyms for the term "in prison". One of the most common synonyms is "incarcerated", which refers to being confined to a jail or prison. Another synonym is "behind bars", which is a figure of speech used to describe being imprisoned. "In custody" is also frequently used to describe being detained or held in prison. Additional terms include "locked up" or "confined", which both refer to being imprisoned or confined to a specific location. In short, there are plenty of words and phrases that can be used in place of the phrase "in prison".