Metalloid
Definition
Elements that behave like a mix of metals and nonmetals: the staircase elements such as silicon and boron that make semiconductors possible.
Technical definition
Metalloids (B, Si, Ge, As, Sb, Te, and sometimes Po or At) are elements along the metal-nonmetal boundary with intermediate properties. They can conduct electricity under some conditions, acting as semiconductors, and form both ionic and covalent compounds.
Example
Silicon's semiconducting behavior, conducting better when heated or doped, is why every computer chip is built on it.
Common confusion
Metalloids are not a formal IUPAC category with fixed membership. Different sources include slightly different elements, but B, Si, Ge, As, Sb, and Te are the core six.