NCIS, formerly known as NCIS: Naval Criminal Investigative Service, is an American police procedural

Police procedural

The police procedural is a subgenre of detective fiction which attempts to convincingly depict the activities of a police force as they investigate crimes. While traditional detective novels usually concentrate on a single crime, police procedurals frequently depict investigations into several...

 drama television series revolving around a fictional team of special agents from the Naval Criminal Investigative Service

Naval Criminal Investigative Service

The United States Naval Criminal Investigative Service is the primary security, counter-intelligence, counter-terrorism, and law enforcement agency of the United States Department of the Navy...

, which conducts criminal investigations involving the U.S. Navy and Marine Corps

United States Marine Corps

The United States Marine Corps is a branch of the United States Armed Forces responsible for providing power projection from the sea, using the mobility of the United States Navy to deliver combined-arms task forces rapidly. It is one of seven uniformed services of the United States...

.

The concept and characters were initially introduced in a two-part episode of the CBS

CBS

CBS Broadcasting Inc. is a major US commercial broadcasting television network, which started as a radio network. The name is derived from the initials of the network's former name, Columbia Broadcasting System. The network is sometimes referred to as the "Eye Network" in reference to the shape of...

 series JAG

JAG (TV series)

JAG is an American adventure/legal drama television show that was produced by Belisarius Productions, in association with Paramount Network Television and, for the first season only, NBC Productions...

 (JAG episodes 8.20 and 8.21).