"The simplest way to define the width of a beam is to choose two diametrically opposite points at which the irradiance is a specified fraction of the beam's peak irradiance, and take the distance between them as a measure of the beam's width. An obvious choice for this fraction is ½ (−3 dB), in which case the diameter obtained is the full width of the beam at half its maximum intensity (FWHM). This is also called the half-power beam width (HPBW)." - Wikipedia. I can see however how this would be confusing.
FWHM, Full-Width at Half Maximum shares definitions across many topics in Physics.
Basically, IF it's described in seconds, it's talking about the graph power vs time (x). Considering you're asking about pulsed lasers, and its FWHM, you're looking for the pulse duration such that the power is at or above half of the maximum.
Document containing more you might find useful here.
Well, do you think this because wikipedia talks about a width? We are talking about electromagnetic waves so the relation between a distance and a frequency (and so a time) is pretty straightforward.
I am asking this because I found the FWHM data of a laser ( a 1064 nm Nd:YAG ) expressed as 6 ns, so I am trying to understand if it actually refers to the pulse duration, or to something else.