Somehow it happened that even in the textbooks they do not give an expanded and deep definition of what it is, a VPN. Like, and so everything is clear: the abbreviation VPN stands for Virtual Private Network, that is, a virtual private network. And why else discuss something? What is a “network” is understandable: at the primitive level, this is the union of two or more nodes with some form of communication so that they can exchange information. Naturally, in the most convenient way and with the support of all the necessary services.
What is “private” is also apparently obvious - not public, therefore private. That is, one in which there is not anyhow who, but only the permitted nodes. If you dig a little deeper, it is this component of the VPN that is the most important, since it determines a number of requirements for this very “particular”.