This term seem to occur at footnotes and bibliographical references.
Fenlon D. Heresy and Obedience in Tridentine, Italy: Cardinal Pole and the Counter Information, Cambridge 1972, ad indicem.
WS Lewis, RT Brown - New Haven [1937-83], ad indicem, 1941
Even through extensive Googling, I am not able to find a proper definition for this term. Neither could I even find a dictionary that could define this.
It seems to be from Latin origin, but its repeated use within English context requires me to ask this question in this site.