Impressionism is a style of art that uses light and color to capture a moment in time, rather than presenting a realistic image. Monet and Renoir were two masters of impressionism.
You can think of impressionism as a technique, in which a painter creates an impression of an object or scene. It's also a major art movement that started in 19th-century France (the word is usually capitalized when referring to that movement). Rather than trying to create true-to-life images, the impressionists attempted to convey reality through gradations of light and generous use of color. In literature, impressionism has a similar meaning, referring to a literary style that does not represent reality realistically, but tries to evoke powerful emotional and sensory responses.