Strictly, any regular testing (monitoring) of application response from outside the edge servers of the delivery infrastructure. This term includes both active (synthetic) and passive (actual site visitor) approaches.
 

Synthetic/active:

• Backbone (primary ISP-based testing), either from individual T1 data centers or LINX locations.

• Cloud-based (e.g., for comparison of relative CDN performance).

• Private-peer locations: any specific location where a vendor test agent has been installed. Typically, these are inside-the-firewall (i.e., intranet locations such as customer-service centers or branch offices), although they could include partner company sites or (in theory) IoT devices or customer test panels (e.g., VIP users of betting and gaming sites).

• Synthetic end-user testing from test agents deployed to consumer grade devices. Depending upon the technology used, these can vary between “true” end users—the dynaTrace Synthetic Last Mile testing is one highly distributed example, private-peer testing, or quasi-end-user testing from consumer grade devices with artificially modelled connection speeds (WebPageTest provides a good open source example of this).

Passive:

• Real User Monitoring (RUM) and other synonyms; see next row.

• The performance analysis of incoming (site visitor) traffic by detection of a variety of navigation and other response related APIs.

• Performance metrics associated with other user device related information (e.g., operating system, screen resolution, mobile device).