A term used in phonology, referring to a universal scale (or scales) of values on which units (segments or classes of segments) can be arrayed, from strong to weak: the behaviour of segments in diachronic or synchronic processes is claimed to be derivable from their rank on the scale. In one approach, for example, velars are assigned a lower rank on this scale (are ‘weaker’) than dentals, which are in turn weaker than labials. The approach aims to determine mechanically, on the basis of a strength scale for segments and for structural positions, the relative probability of any segment occurring in any position, e.g. the hypothesis that strong segments will dominate in strong positions, and weak segments in weak positions.