What is meant by nuclear reactor?

It is a reactor, where atomic nuclei are either combined (fusion) or split (fission), with the consequent release of energy .

That great big bright yellow UFO ( unidentified flying object) in the sky , the SUN is a giant nuclear reactor, whereby hydrogen nuclei are fused together to form helium nuclei. , with the consequent release of energy ; electromagnetic waves( heat, radio waves, UV waves , light etc.,)

If we could see inside a nuclear reactor on Earth it would just look the same, however, nuclear reactors on Earth are just used to collect heat, for electric generation.