noun which may take the genitive case particle 'no', adverb
practically; in effect; de facto
Used to describe something that is essentially the case, even if not officially or formally so. Often modifies nouns with の or functions adverbially.
彼は実質上のリーダーだ。
He is the de facto leader.
この計画は実質上、失敗だ。
This plan is, in effect, a failure.
Compound of 実質 (substance, essence) and 上 (above, on), literally 'on the substance', giving the meaning 'in essence' or 'practically'.