Soft-c Definition

noun

(linguistics) In many languages, a letter "c" which is not pronounced as a velar stop (/k/, like English "k") but as a sibilant (/s/, like English "s") or an affricative (/tʃ/, like English "ch").

Wiktionary
In English, the "c" sound in "cent", "central" and "circuit" as distinct from the hard c in "cat" and "cabin".
Wiktionary
In Italian, the "c" sound in "certo" as distinct from the "c" sound in "caffè".
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Examples.
Wiktionary