In chromatography, when substances are eluted and detected by a detector (Ultra-violet, or Visible wavelenght, or Refractive Index, or Fluorescence, or Light-Scattering Detector, etc.) the resulting peaks have the Gaussian distribution (like a bell-curve).

Peak width is actually calculated by the difference in time of a line drawn parallel to the baseline and intersects the peak of interest. W = t2 - t1.
From this definition, peak width can be measured at baseline, or at half-height, or at 5% peak height, or 10% peak height.

Normally, sharp peaks (with width <0.3 min at baseline) are desirable.If peaks are wide, the separation is not perfect, resulting in a low resolution.

Here is a link (url) to more info, provided by LC Resources (Calif., USA)
http://www.lcresources.com/wiki/index4875.html?title=Main_Page