Sibilance

Sibilance is the name given, at least in the study of literature, to the alliteration, or repetition, of '-s-' sounds. In phonetics - the study of the sounds of language - it means simply 'sounds like '-s-', whether they are repeated or not.

The noun sibilant is the technical term for any one of the sounds that produce sibilance - in English, principally the sounds '-s-' and '-z-'. (Note that these sounds can be produced by other letters, like '-c-'.) The word sibilant is also an adjective which describes the 's' sound.