Definition
Transport Layer Security is the cryptographic protocol that secures most of the modern internet. It sits between an application and TCP, providing encryption (the contents are unreadable to eavesdroppers), authentication (each side knows it is talking to the right counterpart), and integrity (any tampering is detected). TLS is what the 'S' in HTTPS stands for, and it underlies secure email (SMTPS, IMAPS), instant messaging, database connections, and SSL VPNs.
Versions
TLS 1.0 (1999) and 1.1 (2006) are deprecated and disabled in current browsers and software. TLS 1.2 (2008) is widely deployed and considered secure when configured with modern cipher suites. TLS 1.3 (2018) is the current version and a substantial improvement: it removes weak cryptographic primitives, simplifies the handshake to one round trip, and mandates Forward Secrecy. Modern VPN deployments use TLS 1.3 wherever possible.
TLS in VPNs
TLS is the foundation of OpenVPN's authentication and key exchange, and of every SSL VPN. WireGuard does not use TLS — it has its own purpose-built cryptographic protocol — but TLS appears in many VPN management interfaces, account portals, and supplementary services. The shared mathematical primitives (ECDHE, AES-GCM, ChaCha20-Poly1305) tie TLS-based VPNs and modern cryptography together.