Green’s Dictionary of Slang

(US prison) solitary or segregated confinement; als attrib.

1962		Ragen & Finston World’s Toughest Prison 808: meditation manor – Solitary.
1971–2		C. Shafer ‘Catheads [...] and Cho-Cho Sticks’ in Abernethy Bounty of Texas (1990) 210: meditation, n. – the guards’ name for solitary confinement.
2000		Other Side of the Wall: Prisoner’s Dict. July 🌐 Meditation: Solitary or segregated confinement.