where is it used commonly?
In the literature of chemistry.
Metalloid
Despite the lack of specificity, the term remains in use in the literature of chemistry.

what is referred to as a metalloid? It is a type of chemical element which has a preponderance of properties in between, or that are a mixture of, those of metals and nonmetals.
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Metalloid [SEP] Introduction
Some periodic tables include a dividing line between metals and nonmetals, and the metalloids may be found close to this line. Typical metalloids have a metallic appearance, but they are brittle and only fair conductors of electricity. Chemically, they behave mostly as nonmetals. They can form alloys with metals. Most of their other physical properties and chemical properties are intermediate in nature. Metalloids are usually too brittle to have any structural uses. They and their compounds are used in alloys, biological agents, catalysts, flame retardants, glasses, optical storage and optoelectronics, pyrotechnics, semiconductors, and electronics.