In chemistry, chemical synthesis is a purposeful execution of chemical reactions to obtain a product, or several products.  This happens by physical and chemical manipulations usually involving one or more reactions. In modern laboratory usage, this tends to imply that the process is reproducible, reliable, and established to work in multiple laboratories.
A chemical synthesis begins by selection of compounds that are known as reagents or reactants. Various reaction types can be applied to these to synthesize the product, or an intermediate product. This requires mixing the compounds in a reaction vessel such as a chemical reactor or a simple round-bottom flask. Many reactions require some form of work-up procedure before the final product is isolated. 
The amount of product in a chemical synthesis is the reaction yield. Typically, chemical yields are expressed as a weight in grams (in a laboratory setting) or as a percentage of the total theoretical quantity of product that could be produced. A side reaction is an unwanted chemical reaction taking place that diminishes the yield of the desired product.
The word synthesis in the present day meaning was first used by the chemist Hermann Kolbe.
Many strategies exist in chemical synthesis that go beyond converting reactant A to reaction product B in a single step. In multistep synthesis, a chemical compound is synthesised though a series of individual chemical reactions, each with their own work-up.  For example, a laboratory synthesis of paracetamol can consist of three individual synthetic steps. In cascade reactions multiple chemical transformations take place within a single reactant, in multi-component reactions up to 11 different reactants form a single reaction product and in a telescopic synthesis one reactant goes through multiple transformations without isolation of intermediates.
Organic synthesis is a special branch of chemical synthesis dealing with the synthesis of organic compounds. In the total synthesis of a complex product it may take multiple steps to synthesize the product of interest, and inordinate amounts of time. Skill in organic synthesis is prized among chemists and the synthesis of exceptionally valuable or difficult compounds has won chemists such as Robert Burns Woodward the Nobel Prize for Chemistry. 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 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
- 2Na + Cl2 → 2NaCl (formation of table salt)
- S + O2 → SO2 (formation of sulfur dioxide)
- 4Fe + 3O2 → 2Fe2O3 (iron rusting)
- CO2 + H2O → H2CO3 (carbon dioxide dissolving and reacting with water to form carbonic acid)
4 special synthesis rules:
- metal-oxide + H2O → metal(OH)
- non-metal-oxide + H2O → oxi-acid
- metal-chloride + O2 → metal-chlorate
- metal-oxide + CO2 → metal carbonate (CO3)
- Beilstein database
- Chemical engineering
- Methods in Organic Synthesis
- Organic synthesis
- Peptide synthesis
- Total synthesis
- Vogel, A.I., Tatchell, A.R., Furnis, B.S., Hannaford, A.J. and P.W.G. Smith. Vogel's Textbook of Practical Organic Chemistry, 5th Edition. Prentice Hall, 1996. ISBN 0-582-46236-3.
- Advanced Organic Chemistry Part B: Reactions and Synthesis Francis A. Carey,Richard J. Sundberg Springer 2013