Someone with a moderate holding of cryptocurrency.
Cryptocurrency investors are sometimes classified according to the relative size of their holdings. Small investors (those with very modest holdings of a given currency, or a small overall portfolio) are sometimes known as minnows or fish. At the other end of the scale, those with especially large crypto holdings are known as whales.
Crypto dolphins sit between these two classifications: they are no longer counted among the very smallest crypto players, but they have also not yet amassed a large enough holding to be counted amongst the biggest.
In contrast to dolphins or minnows, crypto whales often have their movements monitored closely by the crypto community. Stories abound about crypto whales moving large sums of money, especially across the Bitcoin blockchain. These stories are often used to illustrate one of the primary advantages of Bitcoin: extremely low transaction fees. Crypto whales are sometimes reported to have moved hundreds of thousands of dollars' worth of Bitcoin for just a few dollars in fees.