A Steemian is the term used to describe a member of the steemit website. www.steemit.com
The term "Whale" was originally used to describe a wealthy Bitcoin trader who has an influence on the market. It then became a widely used term for any wealthy cryptocurrency trader.
On Steemit a whale is a user who has a lot of SP (STEEMPOWER) and therefore more voting power.
The term 'Dolphin' is used to describe a cryptocurrency user who does not have as much revenue as a whale but has more revenue than a minnow. This means a dolphin has some influence unlike a minnow. A dolphin is less influenced by a whales activity
As well as describing a small fish, the term "Minnow" also means a small or insignificant person or organization.
In cryptocurrency a Minnow is a is a trader with very little revenue. A minnow has very little or no influence on the market. minnows often follow the herd so are dictated by the whales influence.
On steemit a minnow is a user who does not own much steem power. As such, the minnow has very little voting power and hardly any influence on the revenue distributed to the poster.