In chemistry, the phrase chemical synthesis appears to have one of two meanings. One meaning of chemical synthesis is the process of creating a chemical compound through a series of physical and chemical manipulations usually involving one or more chemical reactions. In modern laboratory usage, this tends to imply that the process is reproducible, reliable, and established to work in multiple laboratories (because, in the bad old days of chemical publication, it was not unheard of for someone to publish a synthetic pathway and "accidentally" leave out a step).

A chemical synthesis usually involves the use of compounds that are known as reagents or reactants. The desired compound produced is called the product. The amount of product in such a synthesis is the yield. Typically yields are expressed as a weight in grams or as a percentage of the total theoretical quantity of product that could be produced.

A chemical synthesis in this broader sense is not restricted to a single kind of chemical reaction or a single step. It may take multiple steps to synthesize the product of interest, and inordinate amounts of time. Skill in this area is prized among chemists and the synthesis of exceptionally valuable or difficult compounds has won chemists the Nobel Prize. If a chemical synthesis starts from basic laboratory compounds and yields something new, it is a purely synthetic process. If it starts from a product isolated from plants or animals and then proceeds to a new compounds, the synthesis is described as a semisynthetic process.

The other meaning of chemical synthesis is narrow and restricted to a specific kind of chemical reaction, a direct combination reaction, in which two or more reactants combine to form a single product. The general form of a direct combination reaction is:

A + B -> AB
where A and B are elements or compounds, and AB is a compound consisting of A and B. Examples of combination reactions include:

2Na + Cl2 -> 2 NaCl (formation of table salt)
S + O2 -> SO2 (formation of sulfur dioxide)
4 Fe + 3 O2 -> 2 Fe2O3 (iron rusting)
CO2 + H2O -> H2CO3 (carbon dioxide dissolving into water to form carbonic acid)