A dolphin to me is someone with 25,000+ SP
...and since steem keeps rising/falling, it's best described as when you upvote someone's post, and you give 50 cents or more when its within the first 15 minutes or so..
A whale on the other hand, is someone that hits a post in the first 15 minutes and sends it instantly to $10.00
This is my non-official description.
Thanks. I start using that same standard myself - it seems more logical that the one I was using.