The term Old Master refers to a group of artists, generally painters, as well as their works. The term is not exact, but roughly encompasses artists and works from the period between 1350 (the dawn of the Renaissance) and 1800 A.D.
That means that artists like Leonardo da Vinci (1452-1519) and Michelangelo (1475-1564) are considered Old Masters, but Claude Monet (1840-1926) and Vincent van Gogh (1853-90) are not.
Likewise, da Vinci’s Mona Lisa (begun around 1503) is an Old Master, while van Gogh’s Irises (1889) is not.
An Old Master designation may signify high quality, but not necessarily, since auction houses use the term chronologically. As a result, auctions of Old Masters can include masterpieces as well as works of lesser quality from the period.