These words all indicates the act of reducing the intensity or severity of something. Lessen is the most informal of these and the most general, applying both to a decrease in intensity and severity. It often suggests the subtracting of a part from a whole, as in its legal use: an appeal to lessen the sentence of the convicted man; a higher court lessened the amount the defendant would have to pay. But the word can apply neutrally to any reduction in degree or quantity: The flight to suburbia lessened the number of middle-class families living in the city itself.

Lighten is metaphorically graphic in referring to a decrease in the weight of something; soften is comparably graphic in referring to a decrease in the harshness, roughness or force of something, both are more often favourable in their implications than lessen . Lighten would apply most naturally where an existing burden is partially lifted, soften most naturally where the full impact of something is prevented from being felt: technology that served to lighten the work load of the factory employee; an attempt to soften the blow by telling her the good news first. In this context, temper refers to making something more moderate, that is, more temperate: a sea breeze that tempered the noonday heat. But another use of the same word gives a meaning that is diametrically opposed, referring to the act of hardening or toughening something: young bodies tempered by rigorous exercise. Thus, unless the context is clear, the use of this word can result in confusion.

The remaining words all emphasize a reduction in severity; they are also alike in pointing to the reduction or removal of something negative or undesirable, rather than the mere subtraction of one amount from another. Allay and assuage can both refer to the calming or satisfying of desire or appetite, but, whereas allay can often suggest a partial lulling or pacifying, assuage might better indicate something approaching satiation: an attempt to allay their hunger by nibbling roots and berries; a huge dinner that assuaged his ravenous appetite. In other uses, furthermore, where assuage remains more strictly tied to the notion of a need that requires satisfaction, allay can function more widely for any appeasing action: reassuring words that assuaged her fear; a codeine tablet to allay the pain. In the former example, allay would suggest less conclusive relief, whereas in the latter assuage would seem out of place.

Alleviate is closest to allay but is even more emphatic about partial relief; it is also restricted to situations in which something is made easier to bear. Thus, it is a more specific and more formal intensification of lighten: early attempts to alleviate conditions in mental hospitals that preceded this century’s thoroughgoing reform in mental care; drugs to alleviate the pain of terminal patients. Mitigate , which comes from a Latin word meaning to soften, can refer more formally to the partial lessening of need, pain or hardship: programmes to mitigate the disruptive effects of slum living; rehabilitative therapy to mitigate the after-effects of strokes and heart attacks; the judge’s decision to mitigate the man’s sentence in light of his previous record. Palliate , in its most common usage today, means to relieve (alleviate ) the symptoms of a disease and thus to mitigate the suffering caused by it.