Dimer

A dimer is a type of molecule made from putting two small molecules together. The two molecules should be the same or very similar. The process of putting these molecules together is called dimerisation. Dimerisation can make covalent bonds, like the combination of cyclopentadiene into dicyclopentadiene, or hydrogen bonds, like the base pairs of DNA.

In polymer chemistry, "dimer" has a broader meaning for any compound where the degree of polymerisation is 2, even if parts of the smaller molecules left the molecule in the condensation reaction.