Second Lieutenant is a junior Company Grade Commissioned officer rank of the UNSC Marine Corps, UNSC Army, and UNSC Air Force; it is the lowest commissioned officer rank and is equal in rank to Ensign in the UNSC Navy. Second Lieutenants are given command of platoons, such as the case of Second Lieutenant Oros in Halo: The Flood.

Second Lieutenant is an entry-level rank for most commissioned officers. A Second Lieutenant usually receives a promotion to First Lieutenant after eighteen to twenty-four months of active service.

In informal settings, Second Lieutenants are referred to simply as "Lieutenant" or "LT."

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