This question can appear trivial, but a lot of sites define OpenVPN like a protocol, other sites define it just like a software. Is it possibile define it as both (i.e. because it uses a custom security protocol by using SSL/TLS at Transport Layer)? What is the correct definition about this?
OpenVPN is both - software (access server and client) and protocol. The protocol uses TLS-encrypted UDP or TCP for transport. It can provide routed or bridged connections. OpenVPN can be used for client VPN as well as for site-to-site VPN.