From a new paper by psychologist Nick Haslam (Melbourne):
[M]any concepts that refer to the negative aspects of human experience and behavior have expanded their meanings. These concepts increasingly extend outward to capture qualitatively new phenomena (‘horizontal’ expansion) and downward to capture quantitatively less extreme phenomena (‘vertical’ expansion). I illustrate these forms of semantic creep by reviewing changes in the concepts of abuse, bullying, trauma, mental disorder, and prejudice over recent decades. In each case, the concept’s boundary has stretched and its meaning has dilated. I argue that this pattern of ‘concept creep’ reflects a dominant moral agenda within social, developmental, and clinical psychology, involving an escalating sensitivity to harm.
Hmm, this sounds familiar, doesn't it? Haslam further explains the two kinds of "expansion" as follows:
The first, ‘vertical expansion,’ occurs when a concept’s meaning becomes less stringently defined, so that it encompasses quantitatively milder variants of the phenomenon to which it originally referred. For example, the concept of obesity would have undergone vertical expansion if the critical body mass index threshold was lowered, thereby swelling the number of people defined as obese. The second form, which I call ‘horizontal expansion’, occurs when a concept extends to a qualitatively new class of phenomena, including application to a new semantic context. For example, the concept of ‘refugee’ has expanded to include people displaced by environmental catastrophe, whereas it originally referred only to those displaced by conflict.