(noun) - In art, an armature is an underlying, unseen, supporting component (usually of wood or metal) for something else. Armatures are useful in sculpture, lost-wax casting (to help make the initial model three-dimensional) and even stop-motion animation puppets.
Think of the chicken wire frame upon which plaster or papier mache strips are affixed in a sculpture, to get a mental visual. An even more dramatic example, designed by Alexandre Gustave Eiffel, is the iron armature inside Frédéric Auguste Bartholdi's Statue of Liberty.
"When this armature has been fixed, the artificer begins to take some fine earth, beaten together with horse dung and hair, as I said, and carefully lays a very thin coating all over which he allows to dry, and so on time after time with other coatings, always allowing each to dry until the figure becomes covered with earth raised to the thickness of half a span at the most." -Vasari on Technique (1907 trans.); pp. 160-161.