Unless otherwise indicated, it is usually defined as the weakest stimulus that can be detected half the time, for example, as indicated by a point on a probability curve.
Signal noise is defined as the presence of extra, unwanted energy in the observational system which obscures the information of interest.
A universal absolute threshold is difficult to define a standard because of the variability of the measurements.
When related to motion in any of the possible six degrees of freedom (6-DoF), the fact that sensory thresholds exist is why it is essential that aircraft have blind-flying instruments.
Sustained flight in cloud is not possible by `seat-of-the-pants' cues alone, since errors build up due to aircraft movements below the pilot's sensory threshold, ultimately leading to loss of control.