inmate

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A resident of a dwelling that houses a number of occupants, especially a person confined to an institution, such as a prison or hospital.

from The Century Dictionary.

• noun One who is a mate or associate in the occupancy of a place; hence, an indweller; an associated lodger or inhabitant: as, the inmates of a dwelling-house, factory, hospital, or prison.
• Dwelling in the same place; residing jointly.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who lives in the same house or apartment with another; a fellow lodger
• adjective rare Admitted as a dweller; resident; internal.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A person confined to an institution such as a prison (as a convict) or hospital (as a patient)
• noun A person who occupies or dwells within a dwelling-house. The word came to be used to refer to temporary inhabitants such as guests in a hotel, students in an on-campus dormitory, patients in a hospital, or prisoners.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a patient who is residing in the hospital where he is being treated
• noun a person serving a sentence in a jail or prison
• noun one of several resident of a dwelling (especially someone confined to a prison or hospital)