sibilants, sibilance

The words sibilants, sibilance sound the same but have different meanings and spellings. Why do sibilants, sibilance sound the same even though they are completely different words?

The answer is simple: sibilants, sibilance are homophones of the English language.

sibilance
  1. :: noun

    The quality of being sibilant: a hissing quality.

sibilants
  1. :: noun

    Plural form of sibilant.

Definitions from Wiktionary, Creative Commons Attribution/Share-Alike License and Wordnik.

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About Homophones

Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.

If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").