Stainless steel, an abbreviation for “stainless acid-resistant steel,” is a type of steel that is resistant to corrosion by air, steam, water, and other weak corrosive media. It is also known as stainless steel if it is resistant to corrosion by chemical corrosive media (acids, alkalis, salts, and other chemical impregnations). Steel that is only resistant to corrosion by weak corrosive media is often called stainless steel, while steel that is resistant to corrosion by chemical media is called acid-resistant steel. This distinction is made because the two types of steel have different chemical compositions. Stainless steel is not necessarily resistant to chemical media corrosion, while acid-resistant steel is generally stainless. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel.

Stainless steel is typically divided into three categories based on its metallurgical organization: austenitic stainless steel, ferritic stainless steel, and martensitic stainless steel. These three categories can be further divided into duplex steel, precipitation hardening stainless steel, and high alloy steel containing less than 50% iron, depending on specific needs and purposes.