Cold fusion refers to nuclear fusion of atoms at conditions close to room temperature, in contrast to the conditions of well-understood fusion reactions such as those inside stars and high energy experiments. Interest in the field was dramatically increased on March 23, 1989 when Martin Fleischmann, then one of the world's leading electro-chemists, and Stanley Pons reported that they had produced fusion in a tabletop experiment involving electrolysis of heavy water on a palladium (Pd) electrode.
They reported anomalous heat production ("excess heat") of a magnitude they asserted would defy explanation except in terms of nuclear processes. They further reported measuring small amounts of nuclear reaction by-products, including neutrons and tritium.
These reports raised hopes of a cheap and abundant source of energy.