BN-256 was an old name for the Barreto–Naehrig(2,254) Weierstrass curve, when it was though to have close to 256 bits of security. It has since been estimated to have at least 90 bits of security (compared to symmetric ciphers) and is now more commonly refered to by BN254, after the number of bits in its prime modulus. Specifically, mclbn256 is the name of the subset of the MCl library containing the support for this curve.