What is the difference between a node and a vertex?

What is the difference (if any) between a node and a vertex? I can't find the answer after looking at countless sites! Even my book doesn't specify it so I am kind of lost!

It is worth mentioning that I am looking for the difference besides the fact that it is called a 'vertex' when used in a graph and a 'node' when used in a tree.

There are no differences between the words Node and Vertex. Even in some books that explain graph theory and graph algorithms they name it as:

Vertex denoted by v, and sometimes it's called nodes also

There are no major nor minor differences between them.

This is mentioned in the book: Data structure and Algorithms with Object Oriented Design Patterns in C#, Bruno R, Preiss.

In "The Practitioner's Guide to Graph Data", the author avoid the term "node/nodes" and only use vertex/vertices and they explain it as below:

...because we are focusing on distributed graphs, and nodes has different meanings in distributed systems, graph theory and computer science.

In distributed systems, a node can be a client, server or peer, while in computer network it can be a computer or a modem. In computer science, as you already point out, it could be used either for graph theory or tree system.

So in the context of graph theory, node and vertex are used interchangeable. But if you would like to make it clear and avoid any misunderstanding, vertex/vertices is the way to go.