When one or more reactants reacts directly, in one step to form products this is called elementary reaction.

A + B --> C

has non-elementary reaction rate equation

-rA = kCACB1/2

The exponent of CA is 1, the exponent of CB is 1/2, for an overall reaction order of 1 + (1/2) = 1.5. Do not let the stoichiometric coefficients from the reaction mislead you. It has to do with the rate equation for a given reaction, not the (net) chemical reaction itself.

A reaction in which an elementary substance displaces and sets free a constituent element from a compound.

A reaction in which an elementary substance displaces and sets free a constituent element from a compound.

Each step in a reaction mechanism is referred to as an elementary step.