Lenition

In linguistics, lenition is a kind of sound change that alters consonants, making them more sonorous (vowel-like). The word lenition itself means "softening" or "weakening" (from Latin lenis = weak). Lenition can happen both synchronically (i.e. within a language at a particular point in time) and diachronically (i.e. as a language changes over time). Lenition can involve such changes as making a consonant more sonorous, causing a consonant to lose its place of articulation (a phenomenon called debuccalization, which turns a consonant into a glottal consonant like [h] or [ʔ]), or even causing a consonant to disappear entirely.

An example of synchronic lenition in American English is found in flapping in some dialects: the /t/ of a word like wait [weɪt] becomes the more sonorous [ɾ] in the related form waiting [ˈweɪɾɪŋ]. Some dialects of Spanish show debuccalization of /s/ to [h] at the end of a syllable, so that a word like estamos "we are" is pronounced [ehˈtamoh]. An example of diachronic lenition can be found in the Romance languages, where the /t/ of Latin patrem ("father", accusative) becomes [d] in Italian padre and [ð̞] in Spanish padre, while in French père and Portuguese pai it has disappeared completely. Along with assimilation, lenition is one of the primary sources of phonological change of languages.

...
Wikipedia