In Roman city planning, a decumanus was an east-west-oriented road in a Roman city, castra (military camp), or colonia. The main decumanus was the Decumanus Maximus, which normally connected the Porta Praetoria (in a military camp, closest to the enemy) to the Porta Decumana (away from the enemy).
This name comes from the fact that the via decumana or decimana (the tenth) separated the Tenth Cohort from the Ninth in the legionary encampment, in the same way as the via quintana separated the Fifth Cohort from the Sixth.
In the middle, or groma, the Decumanus Maximus crosses the perpendicular Cardo Maximus, the primary north-south road that was the usual main street. The Forum is normally located close to this intersection of the Decumanus Maximus and the Cardo Maximus.
Read more about Decumanus Maximus: Examples