The term "Impressionism" was coined by the French press as a deragatory nickname for a school of French painters who had their first exhibition in 1874. The name is based on two factors. The first is that the paintings looked sketchy and incomplete, like a faintly remembered first impression. The second, and less known factor (some experts say the real factor, is because of a painting done by Claude Monet (a founsing memeber of the school), Impression: Sunrise (1873). This Monet work was (and is) the single best embodiment of the Impressionist movement, and so the school was named for it.
Impression: Sunrise (1873) is currently housed in the Musée Marmottan in Paris.