A painting on a plank of wood or multiple planks joined together.  Most surviving European paintings from the 13th to the early 16th century are panel paintings. Two types of wood were most commonly used in panel paintings: poplar in Italy and oak in northern Europe. Video here.

Duccio di Buoninsegna. Madonna and Child. Tempera and gold on panel. Ca. 1290-1300. Metropolitan Museum of Art, New York.