Metaphysical paintings are ones whose subject is architecture (classical, rather than modern) done with exaggerated or unusual perspective and imagination. In addition, there are no people at all in the painting, and incongruous elements are often added. (In the latter Metaphysical painting had an influence on Surrealism.)
The term was first used around 1910--20 to describe the works of the Italian artists Georgio de Chirico and Carlo Carra. The term comes from the Italian Pittura Metafisica.