I’ve spent 5 minutes trying to find out from google and I’m still none the wiser. The best I could find was the defintion of residual from a medical dictionary which was
residual
[rizij′o̅o̅·əl]
Etymology: L, residuum, remainder
1 adj, pertaining to the part of something that remains after an activity that removes the bulk of the substance.
2 n, (in psychology), an aftereffect of an experience that influences latent behavior.