Page "Chemical reaction" Paragraph 18

The elementary reaction is the smallest division into which a chemical reaction can be decomposed to, it has no intermediate products.

Most experimentally observed reactions are built up from many elementary reactions that occur in parallel or sequentially.

The actual sequence of the individual elementary reactions is known as reaction mechanism.

An elementary reaction involves a few molecules, usually one or two, because of the low probability for several molecules to meet at a certain time.