Rhetorical functions in academic writing: Defining

Lava is the name applied to the liquid rock material, or magma, when it reaches the surface, as well as to the solid rock formed by consolidation due to cooling. The temperature of lava as it comes to the surface may exceed 2000°F, for copper wire with a melting point of 2200°F was melted in the lava from Vesuvius, and at Kilauea a temperature of 2300°F. has been observed.
This earth of ours by Victor T Allen, p. 3

The sediment deposited by a stream is called alluvium.
This earth of ours by Victor T Allen, p. 97.

Diseases and symptoms
A disease is normally defined as an abnormal condition of the body that has a specific cause and characteristic outward 'signs' and symptoms. Technically speaking, a 'sign' is considered to be an indication of a disease that is noticed by the doctor but not by the patient, while a symptom is something felt or perceived by the patient himself - but this distinction is often blurred in ordinary conversation.

Most metals are malleable; they can be hammered into flat sheets; nonmetals lack this quality. Some metals are also ductile; they can be drawn out into thin wires; nonmetals are not usually ductile.

Inquiry into earth and space science, by William J Jacobson, p 104-105.