A synthetic, complex, ester polymer, widely used in making synthetic fabrics.
The most common one is polyethylene terephthalate (PET), the same material used in water and soft drink bottles.
In clothing it makes a resilient fiber that is smooth, springy, and resistant to creasing, shrinking, stretching, mildew, and abrasion.
It is hydrophobic (water-repelling) and dries quickly.
Polyester, the miracle substance with the unpronounceable name.