Nickname implies it's unofficial, but S3 is very much the product name too:

https://aws.amazon.com/s3/faqs/

"Simple storage service" is used once. "S3" is used throughout.

While you’re technically correct, for all intents and purposes it is called S3 even by AWS themselves.

When I was a student we had a Facebook group to share information, and one angry guy ranted that the correct shortening of "Mathematical Analysis" is not, in fact, "anal", as we were used to say

and EC2 stands for "Elastic Compute Cloud". but no one remembers that.