from the paper(o) in citation 3 from the linked wiki article(i):

The terms "eyeballs in" "eyeballs out," and "eyeballs down" correspond to acceleration fields AX, -Ax, and AN, respectively, where AX, -Ax, and AN refer to the direction of acceleration forces measured in the conventional airplane body-axis coordinate system.

AN acceleration factor, ratio of acceleration force to weight, positive when directed upward along spinal axis (i.e., from seat to head )

AX acceleration factor, ratio of acceleration force to weight_ positive when directed forward transverse to spinal axis (i.e., from back to chest)

(o) https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/199802...

(i) https://en.m.wikipedia.org/wiki/High-G_training