Paul Smith, Impressionism Beneath the Surface (New York : H.N. Abrams, 1995)

In April 1874, Claude Monet  showed  Impression,   Sunrise  (1873) at  an  exhibition by  a group   calling  itself "Painters, Sculptors,  Engravers  etc. Inc."  This was the first of what we now  know  as the  Impressionist exhibitions.  Monet's  painting and its eccentric  title helped  give Impressionism its name and, indeed, for  many  people,  have  come to  exemplify  Impressionism in general. The critic and friend of the Impressionists, Theodore Duret,  wrote  in 1878:

"If  the  word  Impressionist was... accepted  to designate a group  of painters, it is certainly the peculiar qualities of Claude  Monet's  paintings which first suggested it.  Monet  is the Impressionist painter par  excellence." Duret then went on to give his reasons:

Claude Monet has succeeded in setting down the fleeting impression which his predecessors had neglected or considered  impossible  to  render  with  the  brush....  No longer  painting merely the immobile and permanent aspect of a landscape, but also the fleeting appearances which the accidents of atmosphere present to him, Monet transmits a singularly lively and striking sensation of the observed scene. His canvases really do communicate impressions.

Claude Monet, Impression, Sunrise (1873)

Edouard Manet, Music in the Tuileries (1862)

Duret was arguing that what made a painter an Impressionist was that he or she pursued transient atmospheric effects, which he seems to suggest were also the painter's  impressions. However, Duret's definition of Impressionism, and the idea deriving from it  that  Monet  was the archetypal Impressionist, are not without  their problems. For one thing, as we shall see, an impression was not  just the record of transient effects of light and atmosphere.  For another, the Impressionists were a group whose members' work was far more diverse than Duret suggests.

This, of course, raises the questions who and what were the Impressionists? One  familiar answer is that  they  were a group of painters,  some of whom  first met  between  1860 and 1862 at an informal studio,  the Academie Suisse, and the rest of whom shared  time  together   between  1862 and 1864 in the studio of the painter Charles Gleyre at the Ecole des Beaux-Arts. Monet, Camille Pissarro, and Paul Cezanne (who met at the Academie Suisse) and Auguste Renoir, Alfred Sisley, and Frederic Bazille (who  met Monet at Gleyre's) are said to have united in opposition to a conservative,  classical training and, under the example of the landscapists Eugene Boudin and Johan Barthold Jongkind, went to paint nature in the open air in a novel, bold, and "sketchy" manner.