Return on Ad Spend (ROAS) is a marketing metric that compares the sales generated by advertising to the corresponding advertising costs. It is used to compare the performance of ads, media channels, platforms, etc.
Depending on needs, the sales attributed to the ads can be specified in terms of incremental or marketing exposed sales. Likewise, the costs can be limited to the direct ad creation and media placement costs or can include other costs such as the salary of in-house and contracted personnel who manage the ad campaign, vendor costs, and affiliate costs. 
Return on Ad Spend (%) = Sales Attributable to Ads ($) / Cost of Ads ($)
- Common Language in Marketing Project, 2022