* Elements: Elements are the simplest forms of matter that cannot be broken down into simpler substances. Examples include iron (Fe), carbon (C), and oxygen (O).

* Compounds: Compounds are formed when two or more elements chemically combine in a fixed ratio. Examples include water (H₂O) and carbon dioxide (CO₂).

* Mixtures: Mixtures are combinations of two or more substances where each substance retains its own properties. The components of a mixture can be physically separated.

Steel is a mixture of iron (Fe) with small amounts of other elements, primarily carbon (C). The exact proportions of these elements vary depending on the type of steel, giving it different properties like strength, hardness, and ductility.

Since steel is a combination of multiple elements that can be varied, it fits the definition of a mixture rather than a pure substance, element, or compound.