Malcolm Shute's answer is right. "Node" is originally a Graph Theory 
term that means the same thing as "Vertex".

In distributed systems the term is often ambiguous. For instance, in a 
context where you run several instances of the software on each 
machine, some people say "node" to mean "process" and others say "node" 
to mean "machine". So a system with 10 machines each running 5 
processes could be seen as a "10-node system" or a "50-node system".

The terms "physical node" and "logical node" are sometimes used to 
disambiguate. The "logical node" represents something that has an 
"address", and identifier in the system.

Some systems even have a concept of a "virtual node": something that 
has an address but does not actually represent a distinct part of the 
system. Think of a computer with several IP addresses, for instance. 
You will find this technique, among other things, for load balancing in 
DHTs which use consistent hashing.

tl;dr: "node" is a rather generic term that can mean lots of things and 
usually people who use it define it somewhere first.