Imprisonment is a legal term.

The book Termes de la Ley

Termes de la Ley

Expositiones terminorum legum Angliae is a book by John Rastell. It, and The Abbreviacion of Statutis , are the best known of his legal works....

 contains the following definition:
This passage was approved by Atkin and Duke LJJ in Meering v Grahame White Aviation Co.

See also Bird v Jones (1845) 7 QB 742, (1845) 115 ER 668, (1845) 15 LJQB 82, (1845) 9 Jur 870, (1845) 10 JP 4, (1845) 5 LT (OS) 406.

Imprisonment without lawful cause is a tort

Tort

A tort, in common law jurisdictions, is a wrong that involves a breach of a civil duty owed to someone else. It is differentiated from a crime, which involves a breach of a duty owed to society in general...

 called false imprisonment

False imprisonment

False imprisonment is a restraint of a person in a bounded area without justification or consent. False imprisonment is a common-law felony and a tort. It applies to private as well as governmental detention...

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Imprisonment is a type of sentence

Sentence (law)

In law, a sentence forms the final explicit act of a judge-ruled process, and also the symbolic principal act connected to his function. The sentence can generally involve a decree of imprisonment, a fine and/or other punishments against a defendant convicted of a crime...

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See also English criminal law#General power of Crown Court to impose a sentence of imprisonment on conviction on indictment.