Fusion

A fusion reaction is when two nuclei join to make a larger one. This is the nuclear version of synthesis reactions.
For example, in the sun, 2 atoms of hydrogen are fused into an atom of helium

This reaction requires incredible amounts of pressure (to force the atoms together). In the case of the sun, that force is gravity. We are unable to force regular hydrogen together with enough force to make sustainable fusion. However, we have discovered that it can be done with less force if we use heavy hydrogen - deuterium or tritium.