psi, sigh, xi

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

The answer is simple: psi, sigh, xi are homophones of the English language.

psi
  1. :: noun

    The 23rd letter of the Greek alphabet. See Table at alphabet.

  2. :: noun

    Parapsychological phenomena or abilities considered as a group.

  3. :: abbreviation

    pounds per square inch

sigh
  1. :: verb-intransitive

    To exhale audibly in a long deep breath, as in weariness or relief.

  2. :: verb-intransitive

    To emit a similar sound: willows sighing in the wind.

  3. :: verb-intransitive

    To feel longing or grief; yearn: sighing for their lost youth.

  4. :: verb-transitive

    To express with or as if with an audible exhalation.

xi
  1. :: noun

    The 14th letter of the Greek alphabet. See Table at alphabet.

  2. :: noun

    See xi baryon.

Definitions from The American Heritage® Dictionary of the English Language, 4th Edition 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").