When a system with a natural vibrating frequency is stimulated by an outside force of the same frequency, the system can be set in vibration. As the frequency of the stimulus closely approaches that of the system, oscillation occurs, which reaches a maximum amplitude at the natural resonant frequency.
Resonance can occur in any vibrating system, including electrical circuits, the sound boxes of musical instruments, rooms, the cavities of the human body, including the vocal tract (formant) and other objects. It can be regarded as a type of natural amplification in that the transfer of acoustic energy is made more efficient, as opposed to an electroacoustic amplifier where energy is added to the system. (Source: Barry Truax - Handbook for Acoustic Ecology CD-ROM Edition. Cambridge Street Publishing, 1999 - CSR-CDR 9901)
Bibliography: ROEDERER, Juan G. (1979). Introduction to the Physics and Psychophysics of Music