Describes a position in which two girls press their breasts (of roughly equal size) against each other.
If there is a noticeable size difference, it is asymmetrical docking instead.

Etymology
The term originates in GaoGaiGar, in which the combined form of Hyouryu and Enryu; Chouryujin, was released through a maneuver called "symmetrical docking". Later, other similar paired mecha performed the same combining maneuver.
In a Super Robot Wars 4koma (see below), the maneuver was likened to the action the tag now stands for.