Definition
An SSL VPN is a VPN that uses TLS — the protocol that underlies HTTPS — to secure its tunnel. The name is historical; almost all such VPNs today actually run TLS, not SSL. The defining property is that traffic looks superficially like ordinary HTTPS web traffic and can run on standard TCP port 443, making SSL VPNs much easier to use behind restrictive firewalls than UDP-based protocols on uncommon ports.
Common Implementations
OpenVPN is fundamentally an SSL VPN: it uses OpenSSL for its cryptography and can run over TCP 443. Many enterprise VPN appliances (Cisco AnyConnect, Pulse Secure, Fortinet SSL VPN) use SSL VPN as their primary protocol. Web-based 'clientless' SSL VPNs go further and provide remote access through nothing more than a browser — useful for occasional access from devices where installing software is impractical, though feature-limited compared with full client VPNs.
Strengths and Weaknesses
Strengths: trivially traverses restrictive firewalls because it looks like HTTPS, well-supported across platforms, mature tooling. Weaknesses: TCP-over-TCP performance issues when running over TCP (a known limitation when both layers do retransmission), heavier than UDP-based protocols like WireGuard. The SSL VPN category is gradually being supplanted by ZTNA and SASE in enterprise environments and by WireGuard-based services in consumer markets.