Comitatenses is the plural in Latin of comitatensis, adjective originally derived from comitatus (that refers to company, although in this military context received a new meaning of 'army of field'), that in turn comes the root comes (partner).
However, historically it acquired a substantive meaning to refer to the troops of the late Roman army (legions and auxiliary troops) that were not merely garrisoned in Limes (the border of the Roman empire which included, among other the river Rhine or the Danube, the geographical areas close to Persia, etc.) as the troops that were called limitanei or ripenses (which means, near the banks), but were mobile troops.
Their role would be to deal with invasions or large-scale military campaigns that could take place anywhere in the empire, so they were composed of soldiers of better military training and equipment that limited them.
In addition, there were second-line troops called pseudocomitatenses, other elite troops under Magister militum called palatinae, and the scholae, personal guard of the Emperor of the Lower Empire.
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