A blockchain has several core functions like storage, networking, consensus, etc. All these are coded and packaged together in the blockchain node software. This software is then distributed to the users willing to participate in the blockchain validation or mining process.
Blockchain node software running on a physical or virtual machine and accepting connections is a blockchain node.
When several nodes connect with each other and exchange information like blocks and transactions, we get a blockchain network. So, in essence, a network is a collection of interconnected nodes.