noun, noun which may take the genitive case particle 'no'
the establishment; the powers that be
Refers to the existing power structure, mainstream authorities, or institutional side, often in contrast with reformers, outsiders, or the general public.
彼は体制側の人間として批判を浴びた。
He was criticized as someone on the side of the establishment.
体制側の意見ばかり
It's a problem that only the establishment's opinions get through.
Compound of 体制 (system, establishment) and 側 (side). The exact historical derivation is uncertain, but the word is conventionally associated with the meaning 'the establishment side'.