What is thermal energy?
Thermal energy is the energy of motion of the atoms or molecules within a quantity of matter. Thus it is the energy associated with the temperature of matter.
There is no opposite of thermal energy. Thermal energy is energy that comes from heat, and therefore comparable to temperature. There is no "opposite of temperature," and there is no "opposite of thermal energy." If an object has high thermal energy, it is hot. The opposite of that would be having low thermal energy, or being cold.