Definition of cloud chamber

1) A device for observing the paths of speeding particiles as vapor molecules condense on them to form foglike tracks.

2) A device which is used to determine which elementary particles are being generated in a nuclear reaction. What happens is that the elementary particles go zipping through a saturated water fog, and the ionized water molecules provide nucleation centers for the condensation of water. From following these tracks, the identities of the particles can be determined.