What is it called when you write a word with the same sounding but different spelling?

There are a few terms that could fit the situation you're describing, depending on the specific context:

Homophones: This is the most general term for words that sound the same but have different spellings and meanings. Examples include "to, too, two" or "there, their, they're."

Homonyms: This term is sometimes used interchangeably with homophones, but technically it's broader. Homonyms can include homophones as well as words that are spelled the same but have different meanings (like "bear" the animal and "bear" to carry).

Heteronyms: This term refers specifically to words that are spelled the same but have different pronunciations and meanings. An example is "bow" (to bend) and "bow" (a knot).

Homographs: This term is used for words that are spelled the same but have different pronunciations and sometimes different meanings. This category includes both homophones and heteronyms.

So, the best term to use depends on the specific situation. If you are simply referring to words that sound the same but are spelled differently, then "homophones" is the most accurate. However, if you want to encompass a wider range of words that have similar spellings but different meanings or pronunciations, then "homonyms" or "homographs" might be more appropriate.