The term “Delta-V” has become widely used as a descriptor of accident severity for automotive crash investigations and safety research, and yet, the term remains poorly defined and vaguely used in too many situations. The literal definition of Delta-V is the velocity vector difference between a pre and post impact. Many current uses of “Delta-V” do not apply the appropriate qualifiers grounded in basic crash dynamic principles, and such lax use of the term can be misleading and, at times, erroneous. These problems are compounded when researchers refer in generic terms to “Delta-V” in national databases such as NASS without describing, or even knowing, what the physical basis for calculating the data entries were. The present analysis shows that essentially all the “Delta-V’s” in the NASS database are based on damage energy methods taken from vehicle damage forensic measurements. Examples of the use of Delta-V in the literature and in field accident reconstruction cases are discussed along with pointing out where errors can and do occur. Also presented is a timely review of basic physical principles of crash dynamics in the context of real-world crash modes and examples.