pH is important in many non-aquaous solutions, but defined differently due to different/no dissociation. I've always been a bit unclear on it; they don't give you a good understanding of it in general chem/analytical chem.
Just like Arckon said - pH is defined for aqueous (and not too concentrated solutions). That means - between other things - that we know how to measure pH of diluted water solutions, but the same technique, if applied to concentrated solutions/non-aqueous solutions is not guaranteed to give reliable results. In other words - if we were both to measure pH of a 0.1 M acetic acid solutions using different electrodes and different ORP meters we will get the same result, but if we were to measure pH of 0.1 M acetic acid in ethanol (using the same devices and electrodes), we would probably get different results, and neither could be called a "correct one". At the same time "acidity" is an important parameter for almost every mixture in which chemical processes take place. There is no such thing as a "universal pH scale", one that would work in every mixture, but we often extend water pH scale to other solutions in a handwavy manner, assuming it will give at least qualitatively correct result. In most cases it does, but there is always a risk it will fail.