noun

• A curtailment or reduction.
  "Must my anxious management, my prudent retrenchments, dear Margaretta's savings, all go!"
• A curtailment or reduction.
• (by extension) Withdrawal.
  "Even then—10 long years ago—our disillusionment over past failures, our revulsion against moralistic posturing, our retrenchment from prescribing what was best for other people, our withdrawal symptoms from the exhilarations of overcommitment—all these argued for the perspective of diversity; for lowering our goals."

noun

• (military, dated) A defensive work constructed within a fortification to make it more defensible by allowing defenders to retreat into and fight from it even after the enemy has taken the outer work.
  "The Half-Moon is a Work always raiſed before the Baſtion's Point, being ſo named from the Lowneſs of its Gorges Cavity, &c. and is to ſecure the Two Faces of the Baſtion; but when the Faces have but a weak Defence from the Ravelin, theſe Works are ſoon made uſeleſs or ruined, and give the Beſieged an opportunity of Lodgment, and may ſerve for Batteries and Flanks againſt the oppoſing Baſtions; however they may be retrenched by Traverſes, yet they will not fail to attack entirely in the Face, or where you have your laſt Retrenchment, alſo that called the Counterguard runs the like Hazard."

noun

• (especially politics) The adoption of a defensive and hostile posture; refusal to compromise, radicalization.
  "Confronted by their practices again and again, these individuals and organizations in the gay community have made token gestures to appease the demands of people of color, while in the long run these confrontations have resulted in retrenchment of attitudes and stereotypes by gay whites."