Metalloid is a term used in chemistry when classifying the chemical elements. The term semimetal is sometimes used synonymously, but this is sometimes defined differently. On the basis of their general physical and chemical properties, nearly every element in the periodic table can be termed either a metal or a nonmetal. However, a few elements with intermediate properties are referred to as metalloids (from the Greekmetallon = "metal" and eidos = "sort"). The line that separates metalloids from nonmetals in the periodic table is referred to as the "amphoteric line".

There is no rigorous definition of the term, but the following properties are usually considered characteristic of metalloids:

• metalloids often form amphotericoxides.
• metalloids often behave as semiconductors (B, Si, Ge).

The concepts of metalloid and semiconductor should not be confused. Metalloid refers to the properties of certain elements in relation to the periodic table. Semiconductor refers to the physical properties of materials (including alloys, compounds) and there is only partial overlap between the two.

The following elements are generally considered metalloids:

• Boron (B)
• Silicon (Si)
• Germanium (Ge)
• Arsenic (As)
• Antimony (Sb)
• Tellurium (Te)
• Polonium (Po)

Some allotropes of elements exhibit more pronounced metal, metalloid or non-metal behavior than others. For example, for the element carbon, its diamond allotrope is clearly non-metallic, but the graphite allotrope displays limited electric conductivity more characteristic of a metalloid. Phosphorus, selenium, tin, and bismuth also have allotropes that display borderline behavior.

In the standard layout of the periodic table, metalloids occur along the diagonal line through the p block from boron to polonium. Elements to the upper right of this line display increasing nonmetallic behaviour; elements to the lower left display increasing metallic behaviour. This line is called the "stair-step" or "staircase". The poor metals are to the left and down and the nonmetals are to the right and up.