The term 'free electron' refers to an electron that is not bound to an atom or molecule and is able to move freely through a material. It is an important concept in the field of physics, as it is related to the electrical conductivity of materials. The crossword solution for this term is 'ION', which is a particle with a single positive or negative charge. Ions are formed when atoms or molecules gain or lose electrons, and free electrons are a type of ION.