What is defined as the act of taking or keeping a person in custody by legal authority?

The act of taking or keeping a person in custody by legal authority is best defined as "arrest." An arrest occurs when law enforcement officers take an individual into custody, typically with the intent to charge them with a crime or to ensure they appear in court. This process involves certain legal protocols, including informing the individual of the reasons for the arrest and their rights, such as the right to an attorney.

While detention refers to holding someone temporarily for questioning or investigation, it does not carry the same legal implications or formal status as an arrest. Surveillance involves monitoring individuals' activities but does not imply that those individuals are in custody. Imprisonment, on the other hand, refers to the confinement of a person following a conviction and is a subsequent process after an arrest and trial. Therefore, arrest is the term that accurately encompasses the action described in the question as it pertains to legal authority taking someone into custody.