Stationary is an adjective meaning "having a fixed or unmoving position," "not moving": "This huge rock is stationary." "For a week, the stock market has been almost stationary." Stationery, a noun, means "writing paper and envelopes." The word has been extended to include office supplies and retail shops that sell stationery and related supplies: "This stationer's supply of stationery is almost exhausted." How to remember which is which? Stationary means "standing" (note the a's). Stationery is used for writing letters (note the e's).
Dictionary of problem words and expressions. Harry Shaw. 1975.