Substructure in an image or time series refers to patterns of activity that may recurr, but are not continuously present.. For example an ECG trace has a complex pattern at each heart beat interspersed with relatively flat areas between – at a high-level this can be seen as simply beats at particular times, or even just a heart rate, but looking at each beat one may detect more subtle heart anomolies.
Used in Chap. 14: page 205