In archival science, a fonds is a group of documents that share the same origin and that have occurred naturally as an outgrowth of the daily workings of an agency, individual, or organization. An example of a fonds could be the writings of a poet that were never published or the records of an institution during a specific period.
Fonds are a part of a hierarchical level of description system in an archive that begins with fonds at the top, and the subsequent levels become more descriptive and narrower as one goes down the hierarchy. The level of description goes from fonds to series to file and then an item level. However, between the fonds and series level there is sometimes a sub-fonds or sous-fonds level and between the series to file level there is sometimes a sub-series level that helps narrow down the hierarchy. 
In the archival science field, it is widely agreed upon that the term fonds originated in French archival practice shortly after the French Revolution as Natalis de Wailly, head of the Administrative Section of the Archives Nationales of France, wrote Circular no. 14, which laid out the idea of fonds as keeping records of the same origin together because prior to this announcement records were classified arbitrarily and inconsistently. In the same Circular no. 14, Wailly also coined the idea of respect des fonds which meant that archivists should leave the arrangement of fonds as it was originated by the person or agency who created the records. However, Luciana Duranti has found evidence of the idea originating in Naples and other places prior to Wailly's Circular no. 14 in 1814. Regardless of origin, respect des fonds spread rapidly across Europe after the publication of the Manual for the Arrangement and Description of Archives, which is commonly referred to as the Dutch Manual, in 1898 and the First International Congress of Archivists in 1910.
The term fonds as created by Wailly was not as precise as it should have been and left a lot of room for interpretation of fonds. Due to this, Prussian archivists issued regulations for the arrangement of archives in 1881. These regulations provided a clearer image of fonds as public records that “should be grouped according to their origins in public administrative bodies,” and this principle was termed Provenienzprinzip, or, as it is more commonly known as today among the English speaking world, provenance. Provenance the belief that archivists should keep a group of records obtained as a unit in itself and not merge it with other documents. Provenance also is sometimes referred to as custodial history as it takes in account the different people or organizations that held these records prior to the archive obtaining them and the way they organized them. Respect des fonds is often confused as being the same as provenance, but the two ideas are primarily different in that provenance refers to maintaining works by specific people or organizations as separate from others, and respect des fonds adds onto this by also maintaining or recreating the original order of the creator. The idea of respect des fonds and fonds transformed the archival world, and it is still in use today.
In modern archival practice, the idea of fonds still exists today, principally in Europe, England and North America. However, the fonds is sometimes changed slightly to suit other archival practices. For example, in England the term archive group is used instead of fonds, and in the United States' National Archives the term record group is preferred. Record groups are often compared to fonds, but in actuality they can be composed of more than one fonds or not even a full fonds. In Australian archival theory, there is recognition of the principle of respect des fonds, but the theory focuses on series as the primary descriptive level and the existence of multiple provenances. Fonds should not be confused with the term collection, which is used for document aggregations assembled based on some shared characteristic by a collector, but it is not created by the collector and it often does not follow provenance.
As archives are increasingly being digitized (scanned and stored on a computer) and moved to an electronic platform, the idea of a fonds existing in an online database is shifting. An electronic catalog does not sort its items in the fonds level of description to follow provenance procedures unless told to do so, and it does not automatically sort the items within in a chronological order to follow respect des fonds practices either. There is also the issue of items that are born digital, which are items that have been created electronically and are not automatically subject to the hierarchy of a physical item. The practice of implementing fonds in an electronic database presents new challenges in keeping a fonds together electronically as well as physically. As Jefferson Bailey puts it, "the database logic is nonlinear and there is no original order because order is dependent upon query." In the digital context, some archives have taken to describing their holdings on a fonds or series level, or if an archive chooses to do a file and item level description, the fonds can be kept together by implementing metadata and ensuring that the metadata has information on the relationships between items to link together the item and its higher level descriptions. Fonds in a digital archive is an issue that will continue to evolve as digital archives continue to evolve, and it remains to be seen how fonds will evolve in this context.