Monofilament means a single strand, the raw material is pushed through a die to produce a strand of line, so whether it's a single polymer or a blend of polymers, it is still monfilament. Braid is made by knitting several strands of monofilament. Almost all mono is copolymer now, utilising other polymers to replace the additives and plasticisers in the old nylon line making it more supple without compromising the strength.
Sunlight, ozone and exposure to oil and grease used to leech out the plasticiser in old nylon turning it brittle in a short period of time, a good copolymer line will resist attack for much much longer.