In Google Adsense, Page RPM stands for "Page Revenue per Thousand impressions".
Here's how Google defines Page RPM:
Page revenue per thousand impressions (RPM) is calculated by dividing your estimated earnings by the number of page views you received, then multiplying by 1000.
Page RPM = (Estimated earnings / Number of page views) * 1000
For example, if you earned an estimated $0.15 from 25 page views, then your page RPM would equal ($0.15 / 25) * 1000, or $6.00.