What is a synthesis reaction in chemistry?

What you are being asked

You need the definition of a reaction type based on what happens to the reactants and products. For a synthesis reaction, the key idea is that several starting substances join to make a single product.

Core definition in plain language

In a synthesis reaction, two or more reactants combine to form one compound. That is why it is also called a combination reaction.

General reaction pattern

A synthesis reaction usually fits this form:

$$A + B \rightarrow AB$$

The reactants ($A$ and $B$) can be elements or compounds, but the product side has one main compound.

Quick example

$$2Na + Cl_2 \rightarrow 2NaCl$$

Sodium and chlorine combine to make sodium chloride, a single compound.