Simply put, it's two molecules of HGH joined together with a weak ('non-chemical') bond.
It can be used as a marker of production/storage quality and effectiveness of the HGH. (the lower the better).
The above applies only to HGH - it gets complicated after that. For example you can have homodimer (two same molecules co-joined - like in HGH) or heterodimer (two different molecules cojoined (like in reverse transcriptase). There are even dimers, which are chemically bonded - usually with disulphide bridges, although other bonds are possible as well.
Thank you for the reply. So when your dimer gets high 3% I believe you stated for batch 31, how significant is the impact on the effectiveness of the GH?
What causes the dimer to vary? Is it part of the production process that they just can't quite get right?