Offset
From NMR Wiki
Offset (Ω, omega; majuscule ) is the difference resulting from substration of the rotating frame frequency (ωrot.frame, omega; minuscule) taken from the Larmor frequencey (ω0). Then, Ω is given by the expression: [Ω = ω0 − ωrot.frame]. The units for offset are in hertz.
References
Keeler, J. (2005) Understanding NMR Spectroscopy, 3rd ed., Wiley.[1][2]