You have read about Jenkins nodes. Cloudbees uses the term.
You have heard of the term "Jenkins slave" servers (which are configured to use their CPU and RAM according to the main Jenkins server's needs). What is a Jenkins node?
A Jenkins node is a Jenkins slave server.
The main Jenkins website defines a node as "A machine which is part of the Jenkins environment and capable of executing Pipelines or Projects. Both the Master and Agents are considered to be Nodes." For more information, see this posting.
From time-to-time in the I.T. field, the term "slave" (e.g., master-slave gates or master-slave flipflops) is shunned as offensive. For example CNN.com had an article that reported that some people were going to abandon the term master-slave in computers in 2002. Here are two articles that discuss the controversy of the teriminology:
You may also want to see this posting.