"Imprisonment" is an uncountable noun and so we never talk about "an imprisonment". You could say, in general: "His punishment was imprisonment". A period of imprisonment can be one day, a lifetime, or anything in between. A specific criminal receives a specific sentence. If the sentence is imprisonment, then this is "a prison sentence". If the sentence is e.g. 2 years in prison/ 2 years of imprisonment, then this is "a two-year prison sentence" or "a prison sentence of two years". A "life sentence" literally means staying in prison for life. In practice, many "lifers" (an informal term) are released after serving shorter sentences.

Life sentence is when he is in prison until death. Imprisonment only lasts for a period of time. For example, there's a a five-year imprisonment and there's also a 6-month imprisonment and so on.