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Laeti (pron.: //), the plural form of laetus (pron.: //) also leti, liti, was a term used in the late Roman Empire to denote communities of barbari ("barbarians", literally "babblers" - of outlandish tongues - i.e. foreigners, people from outside the Empire) permitted to, and granted land to, settle on imperial territory on condition that they provide recruits for the Roman military. The term laetus is of uncertain origin, but most likely derives from a Germanic word meaning "serf" or "half-free colonist". Other authorities suggest the term was of Latin, Celtic or even Iranian origin.
Laeti may have been groups of migrants drawn from the tribes that lived beyond the Empire's borders. These had been in constant contact and intermittent warfare with the Empire since its northern borders were stabilized in the reign of Augustus in the early 1st century. In the West, these tribes were primarily Germans, living beyond the Rhine. There is no mention in the sources of laeti in the Eastern section of the Empire. Literary sources mention laeti only from the late 3rd and 4th centuries.
Notitia Dignitatum 
Much of our information on laeti is contained in the Notitia Dignitatum, a document drawn up at the turn of the 4th to 5th century. The document is a list of official posts in the Roman Empire, both civil and military. It must be treated with caution, as many sections are missing or contain gaps, so the Notitia does not account for all posts and commands in existence at the time of compilation. Furthermore, the lists for the two halves of the Empire are separated by as much as 30 years, corresponding to c. 395 for the Eastern section and c. 425 for the West, and may include deployments from as early as 379. Therefore not all posts mentioned were in existence at the same time, and not all posts that were in existence are shown.
The surviving Notitia mentions laeti settlements only in Gaul - and even the two lists of laeti prefects extant are incomplete. But the Notitia suggests that laeti settlements may have existed in the Danubian provinces also. The list of praefecti laetorum in Gaul contains prefects for the Lingones, Nervii and Batavi.
List of known laeti settlements 
Title XLII of the Western part contains a list of praefecti laetorum in Gaul.
- Batavi and Suevi at Baiocas and Constantia, Lugdunensis II
- Suevi: Ceromannos and another, unknown location in Lugdunensis III
- Franks: Redonas, Lugdunensis III
- Teutoniciani: Carnunta, Lugdunensis IV
- Suevi: Arumbernos (Auvergne), Aquitanica I
- : dispersed over Belgica I
- Acti: Epuso, Belgica I
- : Fanomantis, Belgica II
- : Nemetacum, Belgica II
- Contraginnenses: Noviomagus, Belgica II
- : Remo and Silvamectum, Belgica II
- Lagenses: near the Tungri, Germania II
- Goldsworthy (2000) 215
- Walde & Hofmann (1965) Bd. 1. A - L. 4. Aufl.
- Neue Pauly-Wissowa Laeti
- Jones (1964) 620
- Mattingly (2006) 238
- Notitia Occ. XLII
- Notitia Occ. XXXIV and XXXV
- Notitia Dignitatum (late 4th century)
- Birley, Anthony (2002), Band of Brothers: Garrison Life at Vindolanda
- Elton, Hugh (1996), Roman Warfare 350-425
- Goldsworthy, Adrian (2000), Roman Warfare
- Goldsworthy Adrian, (2005), The Complete Roman Army
- Jones, A. H. M. (1964), Later Roman Empire
- Mattingly, David (2006), An imperial possession: Britain in the Roman empire
- Neue Pauly-Wissowa
- Walde, A. and Hofmann, J.B. (1965), Lateinisches etymologisches Wörterbuch.