Something, usually molecules, made up of two parts.
A number of important proteins are made up of two or more separate proteins. The ones made up of two proteins are dimers. If they are made up of two identical proteins, they are homodimers. If they are composed of two different proteins, they are heterodimers.
Molecules made up of more than two, but not a lot, of subunits are usually referred to as oligomers. Molecules made up of a lot of subunits are referred to as polymers.
For smaller oligomers, you can be more specific:
An individual subunit of a dimer,oligomer, or polymer is called a monomer.
I can't think of a well known dimer at the moment, but there are lot's of ones I know about that you'd know about if you were an immunologist or cell biologist. The T-cell receptor comes to mind...