Consonant cluster reduction
From Teflpedia
Consonant cluster reduction, often cluster reduction is a phonological feature in which the sounds of consonant clusters are reduced.
For example, in English, the word "fifths" is pronounced /ˈfɪfθs/ in careful speech, but this is likely to be reduced to /ˈfɪθs/ in rapid speech.
Historical consonant reduction may leave silent letters present in spelling, e.g. "lamb", etc.
