Traditionally, noise in seismic data processing is regarded as unwanted records in seismic data (wavelets) acquisition. The aim of seismic data processing is to increase the signal to noise (SN) ratio.
So, what is the signal? The signal is a record of a seismic wavefront from source to receiver with a single reflection; anything other than that is noise. Well, this is true to a great percentage in reality; but even if we remove everything other than a single primary reflection for the wavefront, there are still other issues (before migration) that can reduce your signal quality as we will look at.