Not really in steem but the crypto community. People who invested in the early crypto days when bitcoin was $1 are considered whales. These people have accumulated a vast amount of bitcoin, and use the alt coins to spread out their wealth since they cannot cash out without paying a huge amount of taxes.

Whales produce a huge amount of trade volume in alt coins and can make them pump and dump.

Edit: Now I get that there is a different definition for a "Steem whale"