= Spectral phase
<Spectral phase> is the argument of a complex spectral amplitude as a function of <frequency>. Writing a pulse spectrum as $\widetilde E(\omega)=|\widetilde E(\omega)|e^{i\phi(\omega)}$ separates its spectral magnitude from the <phase> function $\phi$.
Two pulses can have identical spectral <intensities> and different temporal shapes because their <frequency> components add with different relative <phases>. A <phase> linear in <angular frequency> shifts the pulse in time; quadratic <phase> adds <group delay dispersion> and commonly broadens an initially unchirped pulse. Measuring spectral <intensity> alone therefore cannot reconstruct the time-dependent <electric field>; phase-sensitive information is also required.
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