Here's a more formal definition:

Synthesis reaction: A chemical reaction in which two or more reactants combine to form a single, more complex product.

Key characteristics:

* Combining: Reactants join together to form a new product.

* More complex: The product is typically more complex than the reactants.

* General form: A + B → AB

Example:

* The reaction of sodium (Na) and chlorine (Cl) to form sodium chloride (NaCl):

* 2Na + Cl₂ → 2NaCl

Let me know if you'd like more examples or information!