1. In chemistry, the manner in which electrons or electric charges are distributed among the atoms in compounds that are planar and symmetric, particularly those with conjugated (alternating) double bonds; the existence of resonance in the latter case reduces the energy content and increases the stability of compounds; such molecular entities have more than one contriubuting structure, each differing only in the distribution of electrons. 2. Sympathetic or forced vibration of air in the cavities above, below, in front of, or behind a source of sound; in speech, modification of the quality (harmonics) of a tone by the passage of air through the chambers of the nose, pharynx, and head, without increasing the intensity of the sound. 3. The sound obtained on percussion of a part that can vibrate freely. 4. The intensification and hollow character of the voice sound obtained on auscultation over a cavity. 5. The natural or inherent frequency of any oscillating system.