The term is an adaptation of the Latin recensio.
In some scholarly traditions it is used as a synonym of ‘redaction’; e.g. in the Oxford English Dictionary the meaning of ‘recension’ under point 2a reads as follows: “The revision of a text (esp. a theological text); a particular form or version of a text resulting from such revision.”. In this sense, a recension is a version of a work which is the result of a process of (intentional) revision by a (group of) scribe(s) or redactor(s).
If a work is preserved in one or more recensions that are substantially different from the original, the editor might decide to edit the different recensions separately, possibly in a synoptic edition. It is sometimes hard to tell where a recension stems from, in some cases it can even be suspected that it might be the result of an authorial revision. Some examples in Andrés Sanz 2008.
Although the terms recension, redaction and version are often used interchangeably, there have been attempts to clarify this terminology, and to use each term for one specific kind or level of revision. Cardelle de Hartmann et al. (2014, p. 232), for example, try to establish a difference of nuances of meaning between recension, redaction and version, in which series the amount and weight of the changes are taken to be increasing.
A distinction is often drawn between closed and open recensions of manuscript traditions, in the sense that in a closed recension, each manuscript has been copied from a single exemplar, while in an open recension there may be more than one exemplar for a single copy.
Other usage
In several modern languages (Dutch, French, German, Italian, Spanish, Swedish...) the word recensio in a modernised spelling is used for a review in a journal or a newspaper. A book review typically goes through the text to be reviewed, so there is again the fundamental meaning of enumeration and examination in this usage. In English this usage of the word has become obsolete (OED meaning 1b: last use in 1872).
References
– Cardelle de Hartmann, Carmen, Darko Senekovic, and Thomas Ziegler. 2014. Modes of variability: the textual transmission of Petrus Alfonsi’s Dialogus. In Petrus Alfonsi and his Dialogus. Background – Context – Reception, edited by Carmen Cardelle de Hartmann, and Philipp Roelli, 227–248. Florence: SISMEL.
– Andrés Sanz, María Adelaida, Jacques Elfassi, and José Carlos Martín, eds. 2008. L'édition critique des oeuvres d'Isidore de Séville: les recensions multiples. Actes du colloque organisé à la Casa de Velázquez et à l'Université Rey Juan Carlos de Madrid (14-15 janvier 2002). Paris: Institut d'études augustiniennes.
In other languages
DE: Rezension
FR: recension
IT: recensione