A quaternion is the quotient of two 3D vectors and is used in 3D graphics and in accelerometer-based sensors to represent orientation or rotational data. While mathematical quaternions are more involved than this, the unit quaternions (or rotational quaternions) used to represent rotation while using WebGL or WebXR, for example, are represented using the same syntax as a 3D point. As such, the type
DOMPointReadOnly) is used to store quaternions.
XRRigidTransform.orientationin the WebXR Device API reference