Principally a pedagogical concept.

As Liversedge pointed out the other day with respect to model constructs, however, that doesn't mean it isn't real.

ETA: It is what exercise physiologists mean when they talk about "lactate threshold" (or just "threshold") in a general/conceptual sense, something that has been the case since I started in the field. The notion of FTP differs only in that it overtly dissociates the idea from lactate and overtly associates it with power. As with LT, however, it is important not to confuse the concept with ways of estimating it.