SYN'THESIS, n. [Gr. to put or set.] 1. Composition, or the putting of two or more things together, as in compound medicines. 2. In logic, composition, or that process of reasoning in which we advance by a regular chain from principles before established or assumed, and propositions already proved, till we arrive at the conclusion. Synthesis is the opposite of analysis or resolution. 3. In surgery, the operation by which divided parts are reunited. 4. In chimistry, the uniting of elements into a compound; the opposite of analysis, which is the separation of a compound into its constituent parts. That water is composed of oxygen and hydrogen, is proved both by analysis and synthesis.