Ad reports are full of jargon, but only a handful of terms matter day to day. Here’s one line each — plus the one concept most people skip and shouldn’t: incrementality.
ROAS (return on ad spend)
= revenue from ads ÷ ad spend. ROAS 4 means $4 back per $1 spent. Caveat: it measures correlation, not necessarily causation — some conversions would happen even without the ad.
CPA (cost per acquisition)
= ad spend ÷ conversions. Lower is better, but read it against order value: high-AOV businesses can tolerate a higher CPA.
CTR (click-through rate) & conversion rate
CTR = clicks ÷ impressions (how appealing the creative/targeting is); conversion rate = conversions ÷ clicks (how well the landing page/product converts). High CTR + low conversion rate usually points at the landing page or an audience mismatch.
Attribution
Which ad touch a conversion gets "credited" to. Different models (last-click, data-driven, …) give different numbers — which is why platform self-reported ROAS tends to look optimistic.
Holdout (control group)
A randomly held-out slice of audience or campaigns that gets NO ads / no optimization, as a control. It’s the key tool for turning correlation into causation.
Incrementality
The conversions the ads actually added — the "ran vs. didn’t-run" difference, measured with a holdout. This is the gold standard for whether ads worked: a campaign with a great-looking ROAS can have low incrementality (those people would have bought anyway).