Synthetic transactions refer to transactions which serve no business value, however with repeating the same connection setup, deliver transaction packets and close the connection you will get proper response time measurement data. Mostly you will get those measurements in milliseconds. I did several troubleshooting sequences in using Synthetic Transactions to determine the root cause of quality lose. A PacketShaper for example uses always 1k transaction data for standardized, artificial traffic, which helps in determine any degradation of expected service quality. Cisco Router are using IPSLA, which does a similar job. Believe it or not, Cisco's IPSLA Command Reference book is 567 Pages  (December 2009) !!! Microsoft Operations Manager is also capable to monitor objects by using Synthetic Transactions.  Sun Management Center can load "Synthetic Transactions Modules" to perform artificial measurements. NetIQ installs Agents to execute basic transactions doing the same thing. They call it AppManager Response Time Modules.

using a combination of Synthetic Transactions and Ping

PacketShaper - some examples can be found on TechieStuff

Sun Management Center 3.5

AppManager Response Time Modules