Military rank; the most junior commissioned officer rank in many armed forces. The exact equivalent varies by country and branch (e.g., army second lieutenant, navy ensign).
He was commissioned as a second lieutenant in the army.
In the navy, 少尉 is sometimes called an ensign.
After graduating, he was appointed second lieutenant.
He was promoted to second lieutenant after finishing training.
少尉 is the lowest commissioned officer rank, whereas 大尉 (captain/lieutenant) is a senior rank two steps above it, and the similar kanji make the two easy to confuse.
From 少 (しょう, 'junior' or 'lesser') + 尉 (い, a military officer rank suffix). The rank system was adopted in the modern period, modeled after Western military structures.