Is Ethereum a network?

Since Ethereum is a protocol, there can be multiple independent "networks" that conform to the protocol without interacting with each other. Your Ethereum account will work across the different networks, but your account balance and transaction history won't carry over from the main Ethereum network.

What is mainnet ETH?

The consensus among peers decides on the inclusion of transactions and the state of the network. Mainnet is the primary public Ethereum production blockchain, where actual-value transactions occur on the distributed ledger. When people and exchanges discuss ETH prices, they're talking about Mainnet ETH.

What is a TESTNET in Ethereum?

A testnet for Arbitrum ↗. A testnet for Optimism ↗. An Ethereum network is a private network if its nodes are not connected to a public network (i.e. Mainnet or a testnet). In this context, private only means reserved or isolated, rather than protected or secure.

Does Ethereum mainnet use proof-of-stake?

Ethereum Mainnet uses proof-of-stake, but this wasn't always the case. The upgrade from the original proof-of-work mechanism to proof-of-stake was called The Merge. The Merge refers to the original Ethereum Mainnet merging with a separate proof-of-stake blockchain called the Beacon Chain, now existing as one chain.