A thermoplastic is a plastic which is liquid above a certain temperature, solid but malleable at some middle range, and can be 'frozen' to a crystalline (glasslike) state below that range. Typically, the 'frozen' state holds at standard temperature and pressure which allows the thermoplastic to be used to make usable objects. The particular characteristic of a thermoplastic is that it can be put through these transitions for an essentially infinite number of cycles without changing its chemical composition or otherwise damaging it. This makes thermoplastics extremely useful for objects intended for recycling - most soda bottles are made of thermoplastics.
As a contrast, a plastic which undergoes a chemical change and permanent hardening when heated or chilled is called a thermosetting plastic. Adding additional heat to a thermosetting plastic will not restore its malleability, but will cause it to decompose or oxidize into other materials.