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mille passus, milia passuum

An ancient Roman unit of distance, literally, “a thousand passus,” where the passus is a unit conceptually equal to a pace of two steps. Somewhere between 1471 and 1485 meters.

Y. Janvier.
Les problèmes de métrologies dans l'étude de la cartographie antique.
Latomus, vol 52, pages 3-22 (1993).

Omert J. Schrier.
Hannibal, the Rhone and the “Island”: Some philological and metrological issues.
Mnemosyne, vol. 59, no. 4, pages 501- 524 (October 2006).

See section 1.3.4.