In short
A metalloid is an element with properties between those of a metal and a non-metal. Boron, silicon, germanium, arsenic, antimony and tellurium are the usual examples.
Metalloids sit along the staircase line that divides metals from non-metals in the periodic table. They look metallic and are often brittle. Many are semiconductors: they conduct electricity better than non-metals but worse than metals, and their conductivity rises when they are heated. This is why silicon is the basis of computer chips. Metalloids are also called semimetals.
Silicon is a shiny grey solid, like a metal, but it is brittle and a poor conductor at room temperature. It forms an acidic oxide, , like a non-metal. This mixture of properties is why it is called a metalloid.
Also searched as: what does metalloid mean, why is silicon called a metalloid, semimetal.