What is the term for a word's meaning becoming more positive over time?

The term for a word's meaning becoming more positive over time is amelioration. It is also called elevation or, in some sources, melioration, and it describes a change in the emotional or social value attached to a word.

Amelioration is one type of semantic change, the historical evolution of word meanings. A word can move from negative to neutral, negative to positive, or simply from positive to more strongly positive. For example, English eager developed from Middle English egre, which was associated with meanings such as “angry” or “acidic,” into its modern sense of enthusiastic or keen.

The opposite process is pejoration, in which a word acquires a more negative meaning. Broadening and narrowing are different: they describe changes in how wide a word's range of reference is, not whether its connotations become better or worse.

Semantic change usually happens gradually. Older and newer senses can coexist for generations, and social groups may deliberately improve a formerly insulting term through reclamation. That makes amelioration a useful term for both ordinary historical shifts and some cases of reappropriation.