The rank of Lieutenant has different meanings in different military formations (see comparative military ranks), but in all cases it is common for it to be divided.
In the United States Army, Air Force and Marine Corps, First Lieutenant is the second-lowest ranking commissioned officer. In contrast to the rank of Second Lieutenant, its attainment requires the completion of training at a formal academy such as West Point or the Air Force Academy.
In the Royal Navy, the rank of Lieutenant is equivalent to that of a Captain in the army. First Lieutenant is not a specific rank, but aboard a ship, one officer would be designated as First Lieutenant (familiarly, "Number One") and would be the effective second in command.