Article: The large external sculptural schemes of Early Gothic churches gave way to more sculpture inside the building, as tombs became more elaborate and other features such as pulpits were sometimes lavishly carved, as in the Pulpit by Giovanni Pisano in Sant'Andrea. Painted or carved wooden relief altarpieces became common, especially as churches created many side-chapels. Early Netherlandish painting by artists such as Jan van Eyck (d. 1441) and Rogier van der Weyden (d. 1464) rivalled that of Italy, as did northern illuminated manuscripts, which in the 15th century began to be collected on a large scale by secular elites, who also commissioned secular books, especially histories. From about 1450 printed books rapidly became popular, though still expensive. There were around 30,000 different editions of incunabula, or works printed before 1500, by which time illuminated manuscripts were commissioned only by royalty and a few others. Very small woodcuts, nearly all religious, were affordable even by peasants in parts of Northern Europe from the middle of the 15th century. More expensive engravings supplied a wealthier market with a variety of images.

Question: What does incunabula refer to?
works printed before 1500