solitary confinement 
noun
The isolation of a prisoner in a separate cell as a punishment: she was kept in solitary confinement for three days...
- It was so called because it was entirely made up of individual cells for the solitary confinement of prisoners.
- He spent nearly three years in prison, sometimes in solitary confinement, sometimes doing hard labour.
- Through his church, he organized a group of men to visit prisoners in solitary confinement.