In short a "vertebral subluxation" is a condition caused by a misaligned vertebra. This can pinch a nerve root, which can cause various conditions throughout the body in addition to pain and muscle tightness. See the "Nerve Chart" on this site.
In 1909 B.J. Palmer wrote that:
"Chiropractors have found in every disease that is supposed to be contagious, a cause in the spine. In the spinal column we will find a subluxation that corresponds to every type of disease. If we had one hundred cases of small-pox, I can prove to you where, in one, you will find a subluxation and you will find the same conditions in the other ninety-nine. I adjust one and return his functions to normal... . There is no contagious disease... . There is no infection... . There is a cause internal to man that makes of his body in a certain spot, more or less a breeding ground [for microbes]. It is a place where they can multiply, propagate, and then because they become so many they are classed as a cause."
-- B.J. Palmer, The Philosophy of Chiropractic, V. Davenport, IA: Palmer School of Chiropractic; 1909
The presidents of at least a dozen chiropractic colleges of the Association of Chiropractic Colleges developed a consensus definition of "subluxation" in 1996. It reads:
"Chiropractic is concerned with the preservation and restoration of health, and focuses particular attention on the subluxation. A subluxation is a complex of functional and/or structural and/or pathological articular changes that compromise neural integrity and may influence organ system function and general health. A subluxation is evaluated, diagnosed, and managed through the use of chiropractic procedures based on the best available rational and empirical evidence."