Zoom Ratio
Also: Erector Ratio
The ratio between a scope's highest and lowest magnification, where a 5-25x optic offers a five times zoom range that drives versatility and cost.
Zoom ratio is found by dividing the top magnification by the bottom, so a 5-25x scope has a five times range and a 3-18x has a six times range. A wider ratio lets one optic cover both close, fast work at the low end and precise long range aiming at the top end, which is why a high magnification spread is prized on a do-it-all rifle. That flexibility is not free, because stretching the range puts heavy demands on the lens train and the mechanics.
Delivering a large ratio means the erector tube assembly must move more glass through a longer travel while holding the image sharp and centered across the whole range. A bigger zoom factor generally raises the price of a scope and can shrink the field of view or eye box at the extremes. At the short, fast end of the market the same idea drives the appeal of an LPVO, where a low-power variable squeezes a wide ratio into a compact package.