A quantified achievement is a resume bullet that backs its claim with a specific, measurable result: "Reduced checkout abandonment from 38% to 22% over two quarters," not "Improved checkout." Numbers transform a bullet from a claim into evidence, and recruiters scan for evidence first.
The four most common dimensions are percentage change, absolute revenue or cost, time or cycle-time savings, and scale (users served, deals closed, team size managed). A strong bullet combines one of these with a strong action verb and enough context to make the number meaningful — "Increased conversions 14%" is OK; "Increased checkout conversion from 2.1% to 2.4% on a $60M funnel" is persuasive.
If you don't have exact numbers, use honest ranges or clear comparisons ("Managed a portfolio between $200-400K annually"; "Cut incident response time from days to hours"). What you want to avoid is vague, unbounded improvement claims — they read as filler.