The words whirred, word sound the same but have different meanings and spellings. Why do whirred, word sound the same even though they are completely different words?
The answer is simple: whirred, word are homophones of the English language.
Simple past tense and past participle of whirr.
in a word In short; in summary: In a word, the situation is serious.
A sound or a combination of sounds, or its representation in writing or printing, that symbolizes and communicates a meaning and may consist of a single morpheme or of a combination of morphemes.
Something said; an utterance, remark, or comment: May I say a word about that?
Computer Science A set of bits constituting the smallest unit of addressable memory.
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").