They are homophones.
We call them homophones.
homophone
Words that sound alike but are spelled differently are called homophones. These are words that have different meanings and often lead to confusion due to their similar pronunciation.
Homographs are words that are spelled the same but have different meanings. Homonyms are words that sound alike but have different meanings. Words that both sound the same and are spelled the same and both homonyms (same sound) Fair, as in country fair and fair as in reasonable for example
Homophones are two words that sound the same but are spelled differently and have different meanings. Examples include "they're," "there," and "their."
Words that are spelled the same but pronounced differently are called heteronyms. These words have different meanings and are pronounced differently based on their context or part of speech.
The homophone for "invitation" is "invitation." Homophones are words that sound the same but are spelled differently and have different meanings.
onomatopoeia
Those words are Homonyms! Because of them, we can have words like sail and sale, right and write, or even words spelled the same like nail and file, and have them sound the same while meaning completely different things.
Heterographs are words that sound the same but have different spelling and meaning. E.g. To, Too, Two.Homophones are words that sound the same but have different meanings. (note could have similar spelling). E.g. Gases or Gasses.Take a look at this for a complete explanation. Especially the chart:http://en.wikipedia.org/wiki/Homophones
palindrome
Palindromes