TWI (Two Wire Interface) or TWSI (Two-Wire Serial Interface) is essentially the same bus implemented on various system-on-chip processors from Atmel and other vendors. Vendors use the name TWI, even though I²C is not a registered trademark. Trademark protection only exists for the respective logo (See upper right corner) and patents on I²C have now lapsed.
Good idea. Would you use i2c as displayed tag?
Good idea. Would you use twi as displayed tag?
TWI doesn't really merit becoming a synonym... It only has six questions, most tagged with i2c anyway, most tagged because of Amtel implementation of TWI.
There is only one question that even warrants the tag in the first place, because it is the only one that addresses any difference between proper i2c and Atmel's TWI.
Personally, those 6 questions should just be manually retagged and forget about the synonym...
As for 555timer, unless they just did it, it's already a synonym. How many questions were tagged as 555timer anyway? https://electronics.stackexchange.com/tags/synonyms shows that it had none. Why add an uncommon synonym when it did not need to be added?