Rhetorical functions in academic writing: Defining
In academic writing, it is often necessary to define your terms.
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.
In this case, the term "lava" is being defined.
The sediment deposited by a stream is called alluvium.
In this example, "alluvium" is being defined.
Diseases and symptoms
In this example, definitions of "disease", "symptom" and "sign" are defined.
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.
The definition is not explicit in this case, but the words "malleable" and "ductile" are defined.
X is ...
X is called ...
X is known as ...
X may be defined as ...
X is a type of Y that/which ...
A type of Y which ... is X