Also referred to as Functional Threshold Power, it is regarded by most athletes as the power that can be sustained for an hour.  In Xert, Threshold Power closely matches that of FTP.  In general, for most athletes, Threshold Power is slightly above FTP.  Inspecting a power-duration curve using the Xert Fitness Signature calculator reveals the difference.  Here’s an example:

Here we have FTP = 280.8 watts and Threshold PowerAlso used as an acronym TP in Xert and sometimes described using the more colloquial term FTP, Threshold Power is... = 282 watts.