The flippening is crypto slang most commonly used for the possibility that ether, Ethereum's native asset, could surpass bitcoin in market capitalization. More loosely, people use “flipping” when one asset overtakes another on a named metric. The comparison needs that metric: a higher token price or trading volume is not the same as a larger market capitalization.
Measuring the comparison
Market capitalization is price multiplied by circulating supply. A comparison should use compatible data sources and the same observation time. The ratio of ether's market cap to bitcoin's market cap reaches 100% when they are equal and exceeds 100% when ether is larger.
Because the assets have different supplies, ether would not need to match bitcoin's unit price to match its market cap.
What a ranking would mean
An overtaking would describe relative market valuation at that moment, not a permanent change or proof of superior technology, security, or investment performance. Other measures, such as transaction activity and fees, answer different questions and require their own definitions. The flippening is a scenario people discuss, not an inevitable event or a timetable for trading.