The relative nature of Bronsted acid-base terminology becomes apparent when we consider substances that can act as either proton donors or acceptors. When two such substances react, how can we predict which will be the proton donor (acid) and which will be the proton acceptor (base)? The answer is that the stronger acid will force the other substance to act as a base. In other words, the substance with the lower pKa will act as the acid, and the other as the base.
In the first example below, methanol is forced to act as a proton acceptor by the strong sulfuric acid. However, in the second example methanol is the proton donor because it is a stronger acid than the amide ion.