Polyester is currently defined as: "Long-chain polymers chemically composed of at least 85 percent by weight of an ester and a dihydric alcohol and a terephthalic acid."

The name "polyester" refers to the linkage of several monomers (esters) within the fiber. Esters are formed when alcohol reacts with a carboxylic acid:

There are, therefore, many possible variations of the generic polyester fiber. Two that are currently produced commercially are polyethylene terephthalate (PET) and poly-1,4, cyclohexylene dimethylene (PCDT). A third polyester fiber, polyethylene oxybenzoate (PEB) was manufactured in Japan during the 1970s and early 1980s under the trade name A-Tell®. Production of this fiber was discontinued, however, because it did not offer enough performance advantages to remain competitive in the textile market.

Polyester is a smooth fiber with an even diameter. The fiber diameter usually ranges from 12-25 micrometers (10-15 denier). The undyed fiber is slightly off-white and partially transparent. The fibers are approximately 35% crystaline and 65% amorphous.

Close up of a polyester fiber: