A panel painting is a painting made on a flat panel made of wood

Wood

Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...

, either a single piece, or a number of pieces joined together. Until canvas

Canvas

Canvas is an extremely heavy-duty plain-woven fabric used for making sails, tents, marquees, backpacks, and other items for which sturdiness is required. It is also popularly used by artists as a painting surface, typically stretched across a wooden frame...

 became the more popular support medium in the 16th century, it was the normal form of support for a painting not on a wall (fresco

Fresco

Fresco is any of several related mural painting types, executed on plaster on walls or ceilings. The word fresco comes from the Greek word affresca which derives from the Latin word for "fresh". Frescoes first developed in the ancient world and continued to be popular through the Renaissance...

) or vellum

Vellum

Vellum is mammal skin prepared for writing or printing on, to produce single pages, scrolls, codices or books. It is generally smooth and durable, although there are great variations depending on preparation, the quality of the skin and the type of animal used...

, which was used for miniatures

Miniature (illuminated manuscript)

The word miniature, derived from the Latin minium, red lead, is a picture in an ancient or medieval illuminated manuscript; the simple decoration of the early codices having been miniated or delineated with that pigment...

 in illuminated manuscript

Illuminated manuscript

An illuminated manuscript is a manuscript in which the text is supplemented by the addition of decoration, such as decorated initials, borders and miniature illustrations...

s and paintings for the framing.
Panel painting is very old; it was a very prestigious medium in Greece and Rome, but only very few examples of ancient panel paintings have survived.