An electron cloud refers to the spatial probability distribution of electrons around an atom, illustrating that electrons do not have fixed positions or velocities as suggested by classical physics. This concept arises from the Heisenberg uncertainty principle, which indicates that precise measurements of an electron's position lead to uncertainty in its momentum. Instead of viewing electrons as distinct particles orbiting the nucleus, they are better understood as a "cloud" of charge, representing the likelihood of finding an electron in a particular area. The term "electron cloud" was popularized by Niels Bohr, although it is considered somewhat outdated in light of modern quantum mechanics, which emphasizes the probabilistic nature of electron behavior rather than fixed orbits.