In many navies, a sub-lieutenant is a naval commissioned or subordinate officer, ranking below a lieutenant. In the Royal Navy (RN) the rank of sub-lieutenant is equivalent to the rank of lieutenant in the British Army and of flying officer in the Royal Air Force (RAF).
In some armies, sub-lieutenant is the lowest officer rank. However, in Brazil, it is the highest non-commissioned rank, and in Spain, it is the second highest non-commissioned rank.
The NATO rank code for the British Royal Navy rank is OF-1 (senior).
