It's commonly used in model terms to mean the nose length or tail length. In the case of the tail it's defined as the distance between the 25% chord points of the two surfaces. For the nose it's the front end to the 25% wing chord point. If you're considering spiral stability then I think it's the distance from the 25% chord point of the wing to the same on the vertical tail or it may be the center of area of the projected wing in side view to the 25% mark on the tail. For this last bit it's been a while since I looked at the particulars.

In other matters ksechlers' distance from a force to pivot is proper. For example the moment arm from the CG to the drag center or CG to neutral point, etc.