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.
The 23rd letter of the Greek alphabet. See Table at alphabet.
Parapsychological phenomena or abilities considered as a group.
pounds per square inch
To exhale audibly in a long deep breath, as in weariness or relief.
To emit a similar sound: willows sighing in the wind.
To feel longing or grief; yearn: sighing for their lost youth.
To express with or as if with an audible exhalation.
The 14th letter of the Greek alphabet. See Table at alphabet.
See xi baryon.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.