Metalloid

A metalloid or semimetal is a chemical element. Metalloids are elements that have the properties of both metal and non-metal elements. It might be shiny, but brittle. It might be dull, but conduct electricity.
The most common semimetal is silicon. Silicon has electrical conductivity between metals and nonmetals. It is a semiconductor.
An approximate way to classify metalloids is to look at the Periodic Table of elements. See the double diagonal line: top left Boron and Aluminium, bottom right Polonium and Astatine. Those diagonals are the metalloids. Some metalloids are not semimetals, and some semimetals are not metalloids.