A weapon of enormous
destructive power possessed by the
UN military and the
JSSDF. Variations of this weapon include: N²
mines, N²
depth charges, N² air charges, N²
bombs, etc. In episode 25', when
Shigeru Aoba worried about the possible use of
BC weapons,
Makoto Hyuuga replied that N ² weapons were to be equally feared.
Source: The End of Evangelion : Glossary contained within The End of Evangelion - Theatrical Program.
Please note that the '2' in N2 is in superscript, as in 'N²', in the original program, however to ease the discovery of this write-up the node title is listed as simply N2.
It is not clear what the N² stands for. It could stand for Non-
Nuclear, as an N² Weapon has the destructive power of a
nuclear bomb without the
radiation.