'Stationary' is an adjective that describes an object that is not in motion. It can describe a physical object or an unchanging condition.When used to describe unchanging conditions, stationary means "stable."
Ex - His car remained stationary after the light turned green.

'Stationery' is a noun that means items used for taking notes or writing letters.It is commonly used to refer to paper with matching logos and designs. This includes: paper, envelopes, pens, pencils, notepads, note cards, etc.
Ex - I bought stationery from the office supply store.