imprison, in prison

Imprison or imprison ( one word ) means to lock up; to confine or to put in jail--
The government of the United States may not imprison anyone without charge for more than 48 hours.
If convicted, he will probably be sentenced to probation; the court might not imprison a first offender.

In prison or in prison ( two words ) means within or inside of ( in ) a jail or prison--
He was in prison for several years, and he's having trouble adjusting.
How long has he been in prison?

Now that you know that, you can say--
"If the judge decides to imprison him, he won't be in prison for more than six months."

Note--"imprisonment" ( one word ) means the state or condition of being confined or in jail--
The sentence is imprisonment for five years.
The phrase "in prison meant" means within or inside ( in ) a jail ( prison ), and what was the sense or value, or what were the consequences ( meant )--
He won an award for his poem about what being in prison meant to him.