Thermal energy is the total energy from the motion of atoms and molecules in a substance. The faster the particles move, the higher the temperature, and the more thermal energy the object has — especially if it also contains many particles.
Thermal comes from the Greek word thermē, meaning "heat." Something that's thermal usually generates, involves, or retains heat. In physics, thermal energy is also related to heat, but it's not the same thing: It refers to the total kinetic energy of a substance due to the motion of its particles; this is different from heat, which is the transfer of energy from a hotter object to a cooler one. Thermal energy describes the energy within an object, while heat describes energy in motion between objects.