The rate at which a site or page acquires new backlinks over time. A natural pattern for a strong launch is a fast initial spike (press, social shares) followed by a long tail; a natural pattern for evergreen content is steady accumulation. Suspicious patterns — a sudden burst of low-quality links from unrelated sites — can trigger Google's spam systems, particularly after core updates focused on link spam. There's no published optimal velocity; the operative idea is "consistent with what an unaided real-world story would produce". Most negative-SEO link-spam attacks attempt to manipulate exactly this signal.