A type of glycoprotein.
When antifreeze glycopeptides are mixed into body fluids, the fluids will not freeze even at subzero temperatures because glycoproteins stick to the edge of any ice crystals that form, preventing further crystallization.
Fish that live in deep-sea and polar waters have antifreeze glycopeptides in their fluids, allowing them to swim without freezing to death.
Naomi Hunter gave Solid Snake a number of shots before his mission. Ostensibly this was to inject him with nanomachines and antifreeze glycopeptides to keep him from freezing, but the injections also contained the FOXDIE virus.