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. All is speculation at this time.
