Life imprisonment or a life sentence was the punishment of remaining in prison for the duration of one's life, rather than execution.
In 2375, while speaking to Ezri Dax in her holding cell, Weyoun said to her, "If you should decide to help us in our search for a quick end to this miserable war, your sentence will be reduced to life imprisonment." (DS9: "Strange Bedfellows")
In 2377, The Doctor favored life sentences for the Nygean prisoners USS Voyager would hand over to the authorities instead of execution. He reasoned that both would ensure the criminals could never harm anyone again. Seven of Nine countered that it would take resources the Nygeans may be unwilling to waste on them. (VOY: "Repentance")