impassable, impassible

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

The answer is simple: impassable, impassible are homophones of the English language.

impassable
  1. :: adjective

    Impossible to pass, cross, or overcome: impassable roads; impassable problems.

impassible
  1. :: adjective

    Not subject to suffering, pain, or harm.

  2. :: adjective

    Unfeeling; impassive.

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").