  We introduce an alternative definition of the main frequency of an ultrashort
laser pulse, the principal frequency $\omega_P$. This parameter is
complementary to the most accepted and widely used carrier frequency
$\omega_0$. Given the fact that these ultrashort pulses, also known as
transients, have a temporal width comprising only few cycles of the carrier
wave, corresponding to a spectral bandwidth $\Delta\omega$ covering several
octaves, $\omega_P$ describes, in a more precise way, the dynamics driven by
these sources. We present examples where, for instance, $\omega_P$ is able to
correctly predict the high-order harmonic cutoff independently of the carrier
envelope phase. This is confirmed by solving the time-dependent Schr\"odinger
equation in reduced dimensions, supplemented with the time-analysis of the
quantum spectra, where it is possible to observe how the sub-cycle electron
dynamics is better described using $\omega_P$. The concept of $\omega_P$,
however, can be applied to a large variety of scenarios, not only within the
strong field physics domain.
