NeoSteel

From The StarCraft 2 Wiki

NeoSteel is an exceptionally durable material created by the Terrans. NeoSteel is most notably used in the CMC Powered Combat Suit of the Terran Marines and the hull of Terran Battlecruisers.

In truth, almost anything made by Terrans probably contains NeoSteel, but the only technological advances that are known to use it are:

• Battlecruisers
• CMC Armors
• Bunkers
• Thors

Regarding its durability, it's known that Hydralisk accelerated spines can penetrate up to 2cm of Neosteel (effectively breaking through the CMC Armour). With this in mind, one would assume that the Hydralisk spines can penetrate even thicker plates of lower quality metal.

NeoSteel is probably related to paristeel in some way. Paristeel is possibly a less expensive and lower quality product than NeoSteel.