SSL (Secure Sockets Layer) is the protocol's original name, developed by Netscape in the mid-1990s. TLS (Transport Layer Security) is its successor — same job, better engineering, different name after the IETF took over standardization in 1999.
Every version of SSL (2.0 and 3.0) is now considered broken and is disabled in modern browsers. What your site uses today, even though the industry still says "SSL certificate" out of habit, is TLS — most commonly TLS 1.2 or TLS 1.3.
Why the old name stuck
"SSL certificate" became the common term before the rename fully took hold, and it stuck the same way people still "dial" a phone. Certificate authorities, hosting panels, and browser padlock tooltips all still use it. Functionally, when someone sells you an "SSL certificate" today, they're selling you a certificate that enables a TLS connection.
Does the difference matter to you?
Only in one practical sense: make sure your server is configured to support at least TLS 1.2, and ideally TLS 1.3, and that SSL 2.0/3.0 and TLS 1.0/1.1 are disabled. Most modern hosting does this by default.
A quick timeline of the actual version history
SSL 2.0 (1995) and SSL 3.0 (1996) were Netscape's original releases. TLS 1.0 (1999) marked the handoff to the IETF and the formal rename. TLS 1.1 (2006) and TLS 1.2 (2008) followed with incremental and then substantial security improvements. TLS 1.3 (2018) is the current, most secure version, with a redesigned handshake removing most of the legacy weaknesses discovered across the earlier versions.
Why 'SSL certificate' isn't technically wrong, just imprecise
Every certificate authority, hosting panel, and browser tooltip still using the phrase "SSL certificate" isn't making an error exactly — the certificate itself (the X.509 file containing your public key and identity) is the same object regardless of which protocol version is actually negotiated when a visitor connects. The imprecision is specifically in implying the outdated SSL protocol is what's providing the encryption, when in practice it's virtually always TLS doing that work today.
How to check which version your own site is actually using
Any SSL scan tool will show you exactly which protocol versions your server currently accepts, and which one a typical modern browser would actually negotiate by default — worth checking periodically, since server software updates occasionally change default supported versions without an explicit, deliberate configuration change on your part.