When it comes to knives the term "Stainless Steel" means that the knife has at least 10.5%-11% Chromium (definition varies depending on the region) and a carbon content of less than ~1.2%.
The Chromium is the core addition that prevents corrosion (rust) and as a result it's the main ingredient to allow us to classify a brew of steel as "Stainless." To be clear, no steel is stainless, it's just that "Stainless Steel" is considerably less likely to rust than a iron knife with no chromium.
If you wish to devour more detail it is included below.
