Master seaman (MS), or matelot-chef (matc) in French, is a non-commissioned member rank of the Royal Canadian Navy, which is between leading seaman (LS) and petty officer 2nd class (PO2). Technically, the rank is actually an appointment, with appointees holding the rank of leading seaman. If demoted, a master seaman will become an able seaman or leading seaman depending on seniority. However, the process to be appointed is very similar to that of a promotion, and holding the appointment of master seaman is a prerequisite to promotion to PO2.
A contributing factor to the confusion of 'appointment' vs 'promotion' is that when promoted/appointed to master seaman, the sailor enters a new pay scale, unlike appointment to able seaman, wherein the sailor has simply entered a new incentive level within the pay scale for ordinary seaman.
The rank of master seaman is sometimes referred to as "master killick", from "killick", the slang for leading seaman. Those personnel junior to the master seaman, however, are advised to refrain from addressing the holder by that term.