The simplest way to define thermoplastic in regards to the types of plastic is to remember that thermo which means heat and plastic which means something that can be re-formed. So thermoplastic material can be heated up and re-formed into another shape.
However because of the properties of plastic in a solid state it has a property called elastic. Which means if you pull on elastic it will stretch and then snap back into its original shape. In the molten sate plastic acts like the definition and can be reshaped and will stay that shape once it is cooled. So the word plastic only is true when defining the substance of plastic when it is in the molten state, once cooled it is correctly called an elastomer.
There are two types of thermo plastic, one is called crystalline the other is called amorphous. The difference between these two plastic is Crystalline has a specific transition temperature where the plastic turns from solid to liquid when it changes state.