Thermoplastic, also known as a thermosoftening plastic, is a polymer that turns to a liquid when heated and freezes to a rigid state when cooled sufficiently. Most thermoplastics are high-molecular-weight polymers whose chains associate through weak Van der Waals forces (polyethylene); stronger dipole-dipole interactions and hydrogen bonding (nylon); or even stacking of aromatic rings (polystyrene). Thermoplastic polymers differ from thermosetting polymers (e.g. phenolics, epoxies) in that they can be remelted and remoulded. Many thermoplastic materials are addition polymers; e.g., vinyl chain-growth polymers such as polyethylene and polypropylene; others are productions of condensation or other forms of polyaddition polymerisation, such as the polyamides or polyester.

Thermoplastics are elastic and flexible due to the strain from manis turban above a glass transition temperature T_g, specific for each one — the midpoint of a temperature range in contrast to the sharp melting point of a pure crystalline substance like water. Below a second, higher melting temperature, T_m, also the midpoint of a range, some thermoplastics have crystalline regions alternating with amorphous regions in which the chains approximate random coils. The amorphous regions contribute elasticity and the crystalline regions contribute strength and rigidity, as is also the case for non-thermoplastic fibrous proteins such as silk. (Elasticity does not mean they are particularly stretchy; e.g., polyamides/Nylons rope and fishing line.) Above T_m all crystalline structure disappears and the chains become randomly inter dispersed. As the temperature increases above T_m, viscosity gradually decreases without any distinct phase change.^{[citation needed]}