Answer: a minor spinal misalignment.
In simplest terms, a subluxation (a.k.a. Vertebral Subluxation) occurs when one or more of the bones of your spine (vertebrae) move out of position and create pressure on, or irritate spinal nerves. Spinal nerves are the nerves that come out from between each of the bones in your spine. The pressure or irritation on these nerves causes the nerve to malfunction and interfere with the signals traveling over those nerves.
How does this affect you? Your nervous system controls and coordinates virtually all the functions of your body. If you interfere with the signals traveling over nerves, parts of your body will not get the proper nerve messages and will not be able to function at 100% of their innate abilities. In other words, some part of your body will not be working properly.
It is the responsibility of the Doctor of Chiropractic to locate subluxations, and reduce or correct them. This is done through a series of chiropractic adjustments specifically designed to correct the vertebral subluxations in your spine. Chiropractic professionals undergo years of training to be expert at correcting these subluxations.