An Ethereum node is a computer connected to internet and running an Ethereum blockchain client.
It may be:
(A) a part of a public Ethereum blockchain (there is only one main net)
(B) a part of a private Ethereum-based blockchain
Latter case is popular with enterprise applications. In such case they do not need to expose their data to the public and they do not face same scalability and transaction costs constraints as the public Ethereum blockchain.
Mic Nort offers solutions and services both for public and private Ethereum blockchains. Mic Nort Plarform is based on Ethereum blockchain.a