Radiofrequency pulse

From NMR Wiki

Radiofrequency pulse or "RF pulse" is a temporary burst of radio waves.

Duration of that burst is called "pulse width", typically abbreviated as PW.

Below is an approximate image of a realistic square 5μs long pulse. Notice that edges of the pulse are not perfect. There is an oscillation of amplitude in the beginning and a somewhat gradual falloff at the end of the pulse. Typically such transient imperfections on the modern hardware are or the order of ~100 ns (nanosecond is 10^-9 seconds)

The "inner" portion of the pulse is filled with the radio-frequency oscillation. That frequency in the NMR literature is called "carrier frequency" and is most often kept constant throughout the experiment (however hardware allows re-setting that frequency during the course of the pules sequence).

Strength of the pulse is more commonly measured in Hertz as 1/(4*PW₉₀), where PW₉₀ is the duration of 90^o pulse at the same power.