in law, term of imprisonment with no definite duration within a prescribed maximum. Eligibility for parole is determined by the parole authority. In this respect, an indeterminate sentence differs from a definite one in that statutes prescribing the latter usually provide for parole eligibility after a specified fraction of the full term—in most countries, from one-half to two-thirds of the original sentence.

Indeterminate sentences were pioneered in the reformatory movement of the last quarter of the 19th century. Release was determined by progress made under the program of training rather than by a judicially established sentence. In current practice, release on parole from an indeterminate sentence is initiated by the recommendation of prison staff personnel, field parole officers, and the parole authorities. Evaluation of the offender's conduct in prison, the nature of his offense, his life history, his prospects in the community, his general personality, and his attitude determine his suitability for parole.