What does ``velocity'' interferometry really measure?*

Optical interferometers (\textit{e.g.}, VISAR and PDV) are commonly used to measure velocity in dynamic compression experiments. Although the basic function of these interferometers is typically straightforward, there are situations where their operation becomes unclear. Both VISAR and PDV are displacement interferometers because they respond to changes in target position; approximate velocities are determined over a finite time duration of the measured signal(s). However, this distinction becomes muddled in various measurements. Interference fringes can be observed despite the lack of any obvious displacement, while in other situations, no interference fringes are observed even when displacement is seemingly evident. This presentation attempts to reconcile the apparent inconsistencies in optical velocimetry. Many of these inconsistencies stem from intuitive, but incorrect, notions of interferometer operation. Ultimately, ``velocity'' interferometry provides a measure of displacement, though the origin of this displacement may be subtle.