A piercing placed under the clavicle (collarbone), usually done in pairs. The piercing is placed so it passes behind the groove of the subclavius muscle, between the trapezius and sternocleidomastoideus muscles. Sometimes the term is used for surface piercings placed over the clavicle or at the ucipital mapillary.
Since it's such a new and rare piercing, there isn't a standard procedure for it yet, but those with very pronounced collarbones are best suited for this piercing. A very long (three inches or so) barbell, usually made of PTFE, is worn in the piercing. Usually, it's pierced at around 10g; even though there's really no chance of rejection, a thicker gauge is used to prevent the cheese cutter effect. However, it must be placed deep enough so it is not constantly pressing on the clavicle, which increases the risk of resorption, or on the clavius muscle, which could cause muscle pain and damage.
Healing time is around 6 to 8 weeks. The entrance and exit points will heal before the interior of the piercing heals, so it's important to be careful with it even after the outside seems healed. Yanking on the piercing will increase bruising and hinder the healing process, so be careful around seat belts, tank tops, long hair, and curious people. Aftercare should involve long sea salt soaks at least twice a day, and rinses with clean water whenever it feels necessary.
Pictures of happy piercees and even some parts of the procedure can be seen at BME's Misc. Deep Piercings page.