Bitcoin dominance shows what percent of the total cryptocurrency market capitalization belongs to BTC.
e.g. when BTC dominance is 38%, it means 38% of all cryptocurrency market cap consists of Bitcoin.
It simply means the market share of that particular coin.
BTC Dominance = Market cap of Bitcoin (÷) Market Cap of Total cryptocurrencies
The calculation is the same for Ethereum and all other coins.
Similar to BTC dominance, Altcoin dominance represents the total market share of Altcoins. Ethereum has still considered an Altcoin, even though it has a huge market cap.
The Dominance rate simply shows the popularity of that particular coin and it's a metric to check how big is that particular coin comparing with BTC.