The words xero-, zero sound the same but have different meanings and spellings. Why do xero-, zero sound the same even though they are completely different words?
The answer is simple: xero-, zero are homophones of the English language.
Forms terms relating to dry- or dryness
Forms terms relating to Xerox photocopying
The numerical symbol 0; a cipher.
Mathematics The identity element for addition.
Mathematics A cardinal number indicating the absence of any or all units under consideration.
Mathematics An ordinal number indicating an initial point or origin.
Definitions from Wiktionary, Creative Commons Attribution/Share-Alike License, 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.
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").