The words cession, session sound the same but have different meanings and spellings. Why do cession, session sound the same even though they are completely different words?
The answer is simple: cession, session are homophones of the English language.
A ceding or surrendering, as of territory to another country by treaty.
Something, such as territory, that is ceded.
A meeting of a legislative or judicial body for the purpose of transacting business.
A series of such meetings.
The term or duration of time that is taken by such a series of meetings.
The part of a year or of a day during which a school holds classes.
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").