The words all ready, already (2 words/1 word) sound the same but have different meanings and spellings. Why do all ready, already (2 words/1 word) sound the same even though they are completely different words?
The answer is simple: all ready, already (2 words/1 word) are homophones of the English language.
ready in every particular; wholly equipped or prepared.
By this or a specified time: The children were already asleep when we got home.
So soon: Are you quitting already?
Informal Used as an intensive: Be quiet already. Enough already.
Definitions from the GNU version of the Collaborative International Dictionary of English, 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").