1. . 
A processed seismic record that contains traces that have been added together from different records to reduce noise and improve overall data quality. The number of traces that have been added together during stacking is called the fold.
See: bins, brute stack, common midpoint, common midpoint method, fold, ground roll, noise, processing, random noise, seismic record, slant stack, stacking velocity, velocity analysis, zero-offset data
2. . 
To sum traces to improve the signal-to-noise ratio, reduce noise and improve seismic data quality. Traces from different shot records with a common reflection point, such as common midpoint (CMP) data, are stacked to form a single trace during seismic processing. Stacking reduces the amount of data by a factor called the fold.