Combat skin
From Halopedia, the Halo wiki
Combat Skin is a Forerunner term used to talk about body armor. In has a classification system from Class 1 to at least Class 12. There may be more higher classes after that 12. MJOLNIR armour is ranked on this scale as Class 2.
Not much is known about Combat Skin. It first comes up in Halo in the level Library when 343 Guilty Spark wonders why the Master Chief had brought "such ineffective weapons" to combat the Flood, and recommends that the Chief upgrades from the MJOLNIR armour, which, according to some Forerunner ranking system, the criteria of which is not gone into, is only Class 2, to a Class 12 Combat Skin, which 343 says is necessary to combat the Flood.
Here 343 is referring to Combat Skin as a type of armour where the MJOLNIR armour is Class 2. However, later 343 talks about the Flood changing the atmosphere, he says "Your Environment Suit should serve you well..." implying that the MJOLNIR armour in Forerunner terms is an Environment Suit.
The differing nomenclature has led some to believe that the two items 343 refers to are different, i.e. your armour is what he refers to as the 'Environment Suit', and your 'Combat Skin' is the Chief's flesh and body. The term "Combat Skin" could refer solely to the shielding technology used by the Mjolnir V armor, an idea that would seem to be given credence by the fact that the monitors seem to be protected by an incredibly strong shielding system themselves, and that the shielding technology the was derived from Covenant technology which in turn was adapted from Forerunner artifacts.
Another theory states that the classes (Class 1 to Class 12) might not be levels of power but rather designations for their purpose. In this sense, a Class 2 suit like the MJOLNIR would be a general combat armor while a Class 12 could be a suit specifically desgined to fight the Flood. However, in Halo: The Flood, the Master Chief upon hearing this, thinks, "If there's a battle suit six times as powerful as the MJOLNIR armor, I'll be first in line to try it on," implying that the Combat Skin classes are indeed ranked in an arithmetic sequence. All is speculation at this time.
