From what I learn-
Nodes are the Selenium instances that will execute the tests that you loaded on the hub.
So this means, on a
node which runs 5 browsers on a particular OS has basically 5 nodes to run.
A Selenium Grid node is a computer system (virtual or physical) that is connected to a Selenium Grid hub. If you have 5 different browsers installed on a node it is still is just one node.
Per node you can specify the maximum number of browser sessions that can be run in parallel on the node, this to manage and prevent the node running out of memory, cpu, network bandwith, etc...and becoming slow or even crashing.
When ever you start a test sessions against the hub, the hub will check the nodes to see if a node has a free sessions slot and if so start the test there. It is possible to specify which browser and operating system the sessions has to run on.
You can find the documentation here: https://github.com/SeleniumHQ/selenium/wiki/Grid2
Definition of node Selenium uses the standard definition of node as on Wikipedia:
A node is a basic unit used in computer science. Nodes are devices or data points on a larger network. Devices such as a personal computer, cell phone, or printer are nodes. When defining nodes on the internet, a node is anything that has an IP address.
Nodes are typically a computer in clustered computer systems.