Migration refers to the natural process by which the body rejects a foreign object. Splinters, thorns, shrapnel or other objects that are embedded in the surface of the skin, will over time be 'pushed' out by the body. Similarly, piercing jewelry may also be rejected by the body and 'pushed out' over time. This can also happen with tattoos in some occasions, such as sclera tattoos.
This process can vary significantly between individuals and at the rate at which it occurs. The area of the body pierced and the depth of a piercing will also affect whether or not it will migrate 'out'. Some piercings are much more prone to migration (surface piercings, scrotal piercings, eyebrow piercings, nipple piercings) while in others migration is rarely or not observed (earlobe piercings, tongue piercings, PA's).
Some people report piercings migrating small distances within the body until stopping and fully healing.