Weak mode (detector alignment)
In tracker alignment, weak modes are coherent misalignments that have little effect on the goodness of fit (e.g. χ2) of a track. This makes them difficult (or impossible) to correct for with track-based alignment, which relies on the dependence of the goodness of fit on the detector alignment.
John Alison's PhD thesis (2012), see PDF page 12 (document page 53, section 5.2.3)