collusive relationship; extremely close (often improper) ties
Refers to an overly cozy, often improper relationship between entities that should remain independent, such as government and industry, or regulators and the regulated. Carries a strong negative connotation of collusion or mutual back-scratching.
The collusive relationship between politicians and corporations has become a problem.
Reforms are needed to sever these cozy ties.
癒着 alone can mean 'adhesion' (medical) or 'collusion' (figurative). 癒着関係 specifically refers to the relationship itself.
なれ合い implies a more informal, often conspiratorial collusion, while 癒着関係 is used for institutional or structural improper closeness.
Compound of 癒着 (ゆちゃく, 'adhesion; collusion') and 関係 (かんけい, 'relationship'). The figurative sense of improper closeness derives from the medical sense of tissues adhering abnormally.