The words air, aire, are, ayre, ere, err, eyre, heir sound the same but have different meanings and spellings. Why do air, aire, are, ayre, ere, err, eyre, heir sound the same even though they are completely different words?
The answer is simple: air, aire, are, ayre, ere, err, eyre, heir are homophones of the English language.
A colorless, odorless, tasteless, gaseous mixture, mainly nitrogen (approximately 78 percent) and oxygen (approximately 21 percent) with lesser amounts of argon, carbon dioxide, hydrogen, neon, helium, and other gases.
This mixture with varying amounts of moisture and particulate matter, enveloping the earth; the atmosphere.
The sky; the firmament.
A giant void; nothingness: The money vanished into thin air.
Obsolete spelling of air.
Second person singular and plural and first and third person plural present indicative of be.
A metric unit of area equal to 100 square meters (119.6 square yards).
A narrow bar of sand or gravel formed by the sea; a sandbank.
Air.
Previous to; before.
Rather than; before.
To make an error or a mistake.
To violate accepted moral standards; sin.
Archaic To stray.
A circuit court held by itinerant royal justices in medieval England.
A person who inherits or is entitled by law or by the terms of a will to inherit the estate of another.
A person who succeeds or is in line to succeed to a hereditary rank, title, or office.
One who receives or is expected to receive a heritage, as of ideas, from a predecessor.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition, from Wiktionary, Creative Commons Attribution/Share-Alike License 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").