VPN (Virtual Private Network) refers to a secure tunnel through public, insecure networks (e.g. the Internet) that permits secured access even from outside to a secure network (e.g. an internal company network) – and hence enables the "virtual" expansion of the secure network size as required.
Strict authentication of all authorised participants prevents unauthorised third parties from accessing the (internal) secure network, whilst strong encryption of the data stream prevents unauthorised access to the data transmitted and hence any intervention in the secured communications.