There's no scientific name for "double joint." It's a common term used to describe joints with an unusually large range of motion, but it's not a recognized medical term.

Here's why:

* Hypermobility: The medical term for increased joint flexibility is hypermobility. This can be due to several factors like genetics, ligament laxity, and connective tissue differences.

* Joint Dislocation: While a "double joint" might seem like a joint can bend in two places, it's actually due to the joint capsule and ligaments being looser than normal, allowing for greater movement. This doesn't mean there are two joints, but rather one joint with increased flexibility.

* No Specific Condition: "Double joint" doesn't describe a specific medical condition. It's a general term used to describe a range of flexibility that might be considered unusual.

In summary, "double joint" is a colloquial term. The scientific term for increased joint flexibility is hypermobility.