Revenue attributed to ads divided by ad spend, usually reported per platform. Platform ROAS is useful for comparing creative and campaigns inside one channel, but it is the most over-trusted number in DTC: every platform claims the same conversion, so the sum of your platform ROAS is fiction. Use it to optimise within a channel, then judge the business on blended MER.
Benchmark. Platform ROAS targets are channel- and margin-specific. Do not set a single ROAS goal across channels, manage blended MER (stage-banded: ~1.5-2.5x at $1-5M rising to 3.5-6x+ at $25M+; 3x+ once past ~$10M) and let each channel's ROAS float to it.