ROAS (return on ad spend)
ROAS is revenue attributed to advertising divided by the cost of that advertising, usually expressed as a ratio or multiple. It is the standard efficiency measure for revenue campaigns, and is only as trustworthy as the attribution model producing the revenue figure.
In practice
A 4x ROAS under last-click and a 4x ROAS under data-driven attribution are different claims. State the model whenever the number travels.
Target ROAS should reflect margin, not habit: a 3x on a high-margin product can outperform a 6x on a thin one.