Some of Degas' work is clearly impressionistic, but you are right in suggesting that very many works are not.
Manet
'Impressionist' as in 'Monet was an Impressionist painter' is an adjective.
Impressionist
Two are definite Claude Monet and P-A Renoir. Edgar Degas is a famous member of the gang, but not strictly an impressionist. Alfred Sisley is, and so is Berthe Morisot. Camille Pissarro was for a time. Vincent Van Gogh would have to be included in the top 3.
Impressionism.
He is seen as an impressionist painter.
Manet was really the bridge between Realism and Impressionism. Edit: Claude Monet painted "Impression, soleil levant" in 1872 wich is the painting that first inspired the name of the Impressionist movement.
Monet
There are thousands of painters of Impressionism, including painters today. Historically, Monet, Manet, Degas, Pissaro, Homer, Macchiaioli, Pissaro, Sisely, Cezanne, Berthe Morisot, and Cassatt, are some of the most famous.
He lived 1840-1926.
Claude Monet
because he was the first painter to become an impressionist