The words yay, yea sound the same but have different meanings and spellings. Why do yay, yea sound the same even though they are completely different words?
The answer is simple: yay, yea are homophones of the English language.
So; by this amount.
Used as an exclamation of pleasure, approval, elation, or victory.
The name of the letter for the y sound in <xref>Pitman shorthand</xref>.
Alternative form of <xref>yea</xref> (yes).
an affirmative
not only so, but
Yes; aye.
Definitions from The American Heritage® Dictionary of the English Language, 5th Edition., from Wiktionary, Creative Commons Attribution/Share-Alike License., from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved. 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").