What is the "tangential" distortion of OpenCV actually tangential to?

Take the position of a point P on the image relative to the geometric image center C. Assume C remains undistorted in the lens produced image, but P is distorted into P'.

If the position of P' is only distorted radially along direction CP, the distortion is said to be radial.

If the position of P' is also displaced tangentially relative to CP (along the tangent to the circle of radius CP), the distortion is said to be tangential.

See for instance "Camera Calibration with Distortion Models and Accuracy Evaluation" (IEEE Transactions On Pattern Analysis And Machine Intelligence), especially Figs.2 & 3 (pg. 968) and Fig.4 (pg.969).