Claude Monet's Impression - Sunrise (1872), the painting that gave the name to the movement. Painted of the sea outside his Le Havre window after lengthy study, and pressed for a name, Monet said he could hardly just call it 'Le Havre.' Impressions - Sunrise it was. Said critic Jules-Antoine Castagnary,
If one wishes to characterize and explain the [new artists] with a single word, then one would have to coin the word impressionists. They are impressionists in that they do not render a landscape, but the sensation produced by the landscape. The word itself has passed into their language: in the catalogue the Sunrise by Monet is called not landscape, but impression.You can see his use of colour briefly explained here.