Extended definition
Backlink from spam, low-quality sites, or sites with patterns penalized by Google. Can attract a manual penalty.
Context and application
Google announced in 2020-2022 that it ignores most toxic backlinks automatically (Penguin 4.0 algorithm built into core algorithm). This doesn’t mean they don’t need to be monitored. Toxicity indicators: suspicious exact-match anchor text (over-optimized), source site with DR < 5, locale without relevance (link from Russian site to Romanian B2B site), manipulative link building pattern (10,000 links in a week). Disavow file remains necessary only for manual penalty notified in GSC. Without notified penalty, toxic backlinks cleanup is more anxiety management than real SEO.
In practice
monitor with Google Search Console + Bing Webmaster, disavow with disavow file if needed.