The words picnic, pyknic sound the same but have different meanings and spellings. Why do picnic, pyknic sound the same even though they are completely different words?
The answer is simple: picnic, pyknic are homophones of the English language.
A meal eaten outdoors, as on an excursion.
Slang An easy task or a pleasant experience.
A smoked section of pork foreleg and shoulder.
To go on or participate in a picnic.
Having a short stocky physique.
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.
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").