Just to add to mckha489's reply above, sapper is the lowest rank (equivalent to private) in the Royal Engineers. The word can be used informally to refer to a member of the Royal Engineers of any rank, just as gunner can refer to anyone in the Royal Artillery, although it too is technically the lowest rank in the RA.
In 1858 after the Crimean War, the Royal Sappers and Miners had amalgamated with the previously officer-only Corps of Royal Engineers and the Corps of Royal Military Artificers, to form what is today known as the Corps of Royal Engineers.