Link velocity
Link velocity is how fast a website gains (or loses) links from other sites over a period, such as new linking sites per month.
Link velocity is how fast a site gains links from other sites. You measure it over a period: new linking sites this month against last month, for example. SEO tools chart it next to the links you lose, so you can see whether your link profile is growing, flat or shrinking.
It's a reading of what's happening, and it's most useful when you compare it with what you did.
What it tells you
A steady rise usually means people keep finding and citing your pages. A sudden jump means something happened, and it's worth finding out what. It could be a story that got picked up, a product launch, or a page that went viral. It could also be spam pointed at you, or a link-building service doing something you didn't ask for.
A sharp drop is worth a look too. It often means a site that linked to you many times changed its templates, or your own pages moved and broke the links.
For example, Fernhill Coffee usually gains about 15 new linking sites a month. In March it gains 90. Most of them turn out to be local news sites covering its new roasting works, which is good news. If they'd been unrelated blogs with the same keyword in every link, it would be a reason to check for a link scheme.
Is fast growth a problem?
Google doesn't publish any rule about how fast a site may gain links. What its spam policies care about is why the links exist. A hundred links from a story people loved are fine. A hundred bought ones aren't, whether they arrive in a week or a year.
The AI search version
AI answers have their own version of this: how quickly new pages start citing you, or how quickly answers start naming you after a launch or a press mention. In AnswerOrbit, Before and after a date in Recommendations shows your numbers either side of the day something happened, platform by platform.