It means, Revenue Per 1000 impressions. So what the difference between eCPM and RPM in adsense?. There is a difference. eCPM is calculated by Page impressions whereas RPM by ad impressions. Remove adsense from inactive pages of your website.
eCPM was the old term, they replaced it by RPM, that's it. Yes, it's how much they pay you for 1,000 page impressions with ads.