I know I put this somewhere in another area of the forum but I think it is very important as designers to know. This knowledge will make you look like your a professional. Please learn the terms of your trade but also learn the correct grammar usage when talking to clients.

The definations below are from Wikipedia.

Stationery has historically meant a wide gamut of materials: paper and office supplies, writing implements, greeting cards, glue, pencil case etc.
(hence when talking to a client you can let them know you can design their stationery [letterhead, business cards, etc]).

Stationary can mean: Not moving (fixed in position, immobile, motionless, static, or at rest). Unchanging (for instance in measure, mode, condition, character).
In statistics and probability: a stationary process.
In mathematics: a stationary point.
In mathematics: a stationary set.
In physics: a time-invariant quantity, such as a constant position or temperature.
In physics: a steady state physical process, such as a vibration at constant amplitude and frequency or a steady fluid flow.
The word or its spelling is sometimes confused with stationery, meaning paper or office supplies.

Every professional designer needs to know the different.

Class over!