Many of the metrics collected by Browser Synthetic are identical to those collected by Browser RUM. Additional synthetic-only metrics are defined below.

Data Downloaded Before Fully LoadedData downloaded before Fully Loaded, in bytes
Data Downloaded Before OnloadData downloaded before Onload, in bytes
DOM Elements per PageviewThe count of DOM elements on the tested page as measured at the end of the test. Includes sub-documents in frames.
Fully Loaded TimeThe time from the start of the initial navigation until 2 seconds of no network activity have passed after Document Complete.  This will usually include any activity that is triggered by JavaScript after the main page loads. In milliseconds and percentiles.
Number of Resources Loaded Before Fully LoadedNumber of resources loaded before Fully Loaded
Number of Resources Loaded Before OnloadNumber of resources loaded before Onload
Speed IndexA calculated metric that represents how quickly the page rendered the user-visible content (lower is better).  More information on how it is calculated is available here.
Start Render Time
The time from the start of the initial navigation until the first non-white content is painted to the browser display, in milliseconds and percentiles.
Visually CompleteThe time when an end-user would determine that the page is visually complete in the viewport. This time is calculated by using the last visual change to the page, in milliseconds and percentiles. The page represents the viewable content (the viewport) in a browser window. The default size for a browser window is 1024 x 768.

The ratio of OKAY sessions (successful) to all sessions, expressed per-million: 0 (for all failed) to 1,000,000 (for all succeeded). The ratio is calculated using the following: (# of successful sessions/# of total sessions) * 1,000,000.

Availability is represented in per-million instead of as a percentage because Controller metrics are given as integers, making expressing the availability as a percentage less precise. For example, although the availability percentage in actuality might be 99.99%, it would only be able to be expressed as an integer such as 100% or 99%.

To convert per-million to a percentage, just divide by Availability (ppm) by 10,000. For example, if Availability (ppm) is 999,800, then Availability (%) is 99.98%. Most Controller UI components display metrics as percentages, but the Metric Browser, which shows "raw data", shows the value per-million.

Billable TimeNumber of billable seconds used in this usage period
Job ExecutionsTotal number of sessions (per browser, per location) for a given job
Pages per SessionNumber of pages per session
Session DurationFor script-based: the time needed to execute the script from the first line to the last line.
For single URL: the time from the first request to the end of the last request 
Session Visual Time (SVT)The sum of all pages' visually complete values (or load event end, when visually complete is missing) for a given session. The time the entire sequence took.