In practice
Length is not the test; usefulness is. A 200-word answer that resolves the query is not thin.
Audit by traffic and conversions: pages with neither, and no strategic role, should be merged, improved or removed.
Why it matters
A large tail of thin pages lowers how the whole site is assessed, not only those pages.
It also spends crawl attention that would otherwise go to the pages you want ranked.
Related terms