Te point in time when Bitcoin (BTC) loses its throne atop the cryptocurrency mountain of market capitalization and hands the reins to Ethereum (ETH). Nobody knows when this will happen or if it will ever happen
Example
“During the 2021 DeFi summer, Ethereum's market cap grew so fast relative to Bitcoin's that crypto Twitter was buzzing about the flippening, though ETH ultimately peaked at about 50% of BTC's market cap.”