(noun) The speed at which a link seller runs away once you discover that your site is penalized because of his links
Link velocity refers to the rate at which links are posted to your site. Proponents claim that altering this "link velocity" is harmful because Google can perceive the change as unnatural. If you think about this, you would only be concerned if your link building activity was genuinely unnatural and you needed a strategy to hide that fact.
To deal with this obvious problem the link velocity proponents attempt to obfuscate the issue with some misdirection and mumbo jumbo - pointing to other considerations like "link direction" or "intention" or "vectors" - nothing that real metrics can even begin to address.
Think Like Google
If you were Google, would you see the rate at which a site acquires links to be important to the relevance of the content? Of course not! Links to fantastic link bait appear randomly, in spurts and stops, ceasing entirely, then coming back in large numbers. And those links come from all kinds of sites, some trustworthy and others not so, with no way to formulate an overall evaluation. There is no logical reason for a search engine to connect this activity to relevance, trust, or any rank creating protocol.
The parties who worship the false god of link velocity are very likely to be link sellers, typically offering garbage links that actually harm your link profile with automated blog comments, profiles, signatures, forum posts, reviews, etc. In 2010, we observed a new Google penalty related to this practice.
The sellers gain traction by pointing you to articles ranking for the term "link velocity" that exist solely for the purpose of creating credibility for the pitch. They always include explanations that seem to cover all the arguments without really doing so. eg "Link velocity isn't just speed, there are more factors that go into assessing link velocity besides how many and how soon." But when you ask the right questions, the answers are not rewarding or really even understandable.
But note that the real intent behind the perpetrators of link velocity claims always relates to some claim of expertise on how to build links that appear natural. And that the solutions all derive from paying some "expert" to build your links.
Like the Tooth Fairy and Santa Claus, the advocates of link velocity untruths depend on naivete.
The Observable Truth
We have the ability to point very large numbers of links at will, and have posted links to hundreds of sites over the years. There has never been an observable correlation with any changes to the rate at which links are posted, only to the amount of PR passed. We encourage moderation in all things, but even in the instances where large numbers were posted quickly we have never seen an issue, even in our experiments where we purposely post links from non-relevant sites.
The most drastic change to a link build would be a full stop - all link work suddenly halted, something the link sellers need to argue against. But this happens all the time in the real world for various reasons, including resource availability, development requirements, or management changes. There is no negative rank consequence (and certainly no penalty) for stopping a link building campaign.
News aggregator websites see huge changes in the rates that links get posted based on the interest in the news. For example, we observed over 80,000 links posted to the story of Bin Laden's death within 24 hours. What do you think that did to link velocity, and how could you ever keep that rate up?
While there is something to be said for attempting to understand the natural state of a website, creating technical sounding terms that create a false reality, or worse, serve up fear of change using bogus arguments, is nothing but a sleazy marketing technique that exploits legitimate concerns as a sales technique. We're calling this one bogus. The moment a potential vendor gives you a "link velocity" argument for buying a link building service, walk away.