The transient, that is the initial attack of a sound (which can be of an extremely short length, e.g., 50ms), contains essential components of that sound and therefore plays a significant role in the perception of timbre.
See also:ADSR [Attack, Decay, Sustain, Release], Envelope
Bibliography: Levinas, MichaŽl (1994). Transients of Attack and Hybrid Sounds: Toward a New Mixity
Verma, Tony S., Meng, Teresa H. Y. (2000). Extending Spectral Modeling Synthesis with Transient Modeling Synthesis