The temperature at which a certain extent of softening occurs is called the softening point for that polymer.
The softening temperature for a particular material is determined by the Vicat test.
In this test, a specimen of the polymer to be tested is placed in a Vicat apparatus. A needle with a cross section area of 1 sq.mm is placed on the specimen with a load of 10 N for the Vicat A test and 50 N for the Vicat B test.
The temperature at which the needle makes an impression 1mm deep is noted as the softening temperature.