The Pople notation is named after the Nobel laureate John Pople and is a simple method of presenting second-order spin coupling systems in NMR.
[1] The notation labels each (NMR active) nucleus with a letter of the alphabet.
The difference in chemical shift, δ, relative to the J-coupling between nuclei mirrors the separation of the letter labels in the Latin alphabet.
For example, AB indicates two nuclei which have similar chemical shifts (Δδ similar to or smaller than J), whereas AX indicates two which lie further apart on the spectrum (Δδ significantly larger than J).
A2B would similarly indicate a spin system containing two equivalent nuclei (A) and a third, inequivalent one (B).