UI is the point when interactions between machines and people happen. It lays down the parameters of a user's interaction with a machine.
Mainnet
A mainnet is a separate blockchain that operates on its own network using its own protocol and technologies.
Coin
Cryptocurrencies known as coins run on separate blockchains and are unrelated to other coins. Such a cryptocurrency's single unit is also known as a coin.
Pair
Exchange cryptocurrencies with each other, like the trading pair BTC/ETH.