Ease

Ease is the difference between your body's actual measurement and the finished measurement of the garment. Zero ease means the garment matches your body exactly - which sounds ideal but is rarely comfortable or even physically workable, since you need at least a little room to breathe, sit, and raise your arms. "Wearing ease" is the minimum most patterns build in for that basic function; "design ease" is whatever's added on top of that for the intended silhouette, and that's the part that actually varies wildly by style.

This is the measurement conversation I have most often with people newer to fitting: a pattern's finished garment measurement being bigger than your body measurement is not a mistake or an insult, it's the point. A fitted bodice block might carry only an inch or two of ease total; an oversized shirt might carry six or eight.

When I'm drafting or adjusting a block, I think about ease separately for each area - bust, waist, hip, upper arm - because a garment can need generous ease through the hip and almost none through the shoulder, and treating "ease" as one single number for the whole garment is how fit goes wrong.