Why is PET sometimes called polyester?

When an organic acid is mixed with an organic base it makes something called an ester.  PET is made with an organic acid - terephthalic acid - and  a base - ethylene glycol.  When we make plastics we use a substance called a monomer and when two monomers react with each other they form a poly(mono)mer, or polymer for short.  So an ester reacting with an ester becomes known as a polyester. When PET is used for fibre or fabric applications, it is usually referred to as "polyester". When PET is used for container and packaging applications, it is typically called "PET" or "PET resin."