An Iso-Cube (or Isolation Cube) is the slang term used in the Judge Dredd stories for a type of prison cell, typically a solitary cell for containment.
Iso-Cubes are a place where sentenced criminals who commit non-capital crimes are assigned. Perps stay there until their sentence is completed, unable to leave. The room is similar to a single-dwelling apartment in a housing block except it is smaller, has less amenities, and has a secure door that only opens from the outside with a durable glass panel on the other side to maintain suicide watch as well as to remind the prisoner they are constantly being watched. Iso-cubes are found within Iso-Blocks which are analogous to prisons located in Mega City One.
Iso-Cubes are mentioned in the 2012 film Dredd multiple times, but never shown.