Ism majrur: the noun after harf jarr
اِسْم مَجْرُورism majrur - noun in jarr
Ism majrur is the noun affected by harf jarr in Arabic grammar.
Simple explanation
اِسْم مَجْرُور (ism majrur) is the noun affected by a حَرْف جَرّ (harf jarr).
Together, they form مُرَكَّب جَارِّي (murakkab jarri).