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Concept
Concepts are abstract ideas that works are about.
OpenAlex indexes about 65k concepts. The Canonical External ID for OpenAlex concepts is the Wikidata ID, and each of our concepts has one, because all OpenAlex concepts are also Wikidata concepts.
Concepts are hierarchical, like a tree. There are 19 root-level concepts, and six layers of descendents branching out from them, containing about 65 thousand concepts all told. This concept tree is a modified version of the one created by MAG. You can view all the concepts and their position in the tree as a spreadsheet here. About 85% of works are tagged with at least one concept (here's the breakdown of concept counts per work).
Each work is tagged with multiple concepts, based on its title and abstract. The tagging is done using an automated classifier that was trained on MAG’s corpus; you can read more about the development and operation of this classifier in Automated concept tagging for OpenAlex, an open index of scholarly articles. You can also implement the classifier yourself using our models and code.
Concepts are linked to works via the concepts property. They’re also linked to authors, venues, and institutions via the x_concepts property, and to other concepts via the ancestors and related_concepts properties.

The Concept object

id

String: The OpenAlex ID for this concept.
id: "https://openalex.org/C2778407487"

wikidata

String: The Wikidata ID for this concept. This is the Canonical External ID for concepts.
All OpenAlex concepts have a Wikidata ID, because all OpenAlex concepts are also Wikidata concepts.
wikidata: "https://www.wikidata.org/wiki/Q14565201"

display_name

String: The English-language label of the concept.
display_name: "Altmetrics"

level

Integer: The level in the concept tree where this concept lives. Lower-level concepts are more general, and higher-level concepts are more specific. Computer Science has a level of 0; Java Bytecode has a level of 5. Level 0 concepts have no ancestors and level 5 concepts have no descendants.
level: 2

description

String: A brief description of this concept.
description: "study of alternative metrics for analyzing and informing scholarship"

works_count

Integer: The number of works tagged with this concept.
works_count: 3078

cited_by_count

Integer: The number citations to works that have been tagged with this concept. Or less formally: the number of citations to this concept.
For example, if there are just two works tagged with this concept and one of them has been cited 10 times, and the other has been cited 1 time, cited_by_count for this concept would be 11.
cited_by_count: 20248

ids

​
Object: All the external identifiers that we know about for this institution. IDs are expressed as URIs whenever possible. Possible ID types:
Many concepts are missing one or more ID types (either because we don't know the ID, or because it was never assigned). Keys for null IDs are not displayed..
ids: {
openalex: "https://openalex.org/C2778407487",
wikidata: "https://www.wikidata.org/wiki/Q14565201",
wikipedia: "https://en.wikipedia.org/wiki/Altmetrics",
mag: 2778407487114027177
}
​

image_url

String: URL where you can get an image representing this concept, where available. Usually this is hosted on Wikipedia.
image_url: "https://upload.wikimedia.org/wikipedia/commons/f/f1/Altmetrics.svg"

image_thumbnail_url

String: Same as image_url, but it's a smaller image.
image_thumbnail_url: "https://upload.wikimedia.org/wikipedia/commons/thumb/f/f1/Altmetrics.svg/100px-Altmetrics.svg.png"

international

Object: This concept's display name in many languages, derived from article titles on each language's wikipedia. See the Wikidata entry for "Java Bytecode" for example source data.
international: {
display_name: {
ca: "Altmetrics",
...
}
}
​

ancestors

List: List of concepts that this concept descends from, as dehydrated Concept objects. See the concept tree section for more details on how the different layers of concepts work together.
ancestors: [
{
id: "https://openalex.org/C2522767166",
wikidata: "https://www.wikidata.org/wiki/Q2374463",
display_name: "Data science",
level: 1
},
{
id: "https://openalex.org/C161191863",
wikidata: "https://www.wikidata.org/wiki/Q199655",
display_name: "Library science",
level: 1
},
// and so forth
]
List: Concepts that are similar to this one. Each listed concept is a dehydrated Concept object, with one additional attribute:
  • score (Float): The strength of association between this concept and the listed concept, on a scale of 0-100.
related_concepts: [
{
id: "https://openalex.org/C2778793908",
wikidata: null,
display_name: "Citation impact",
level: 3,
score: 4.56749
},
{
id: "https://openalex.org/C2779455604",
wikidata: null,
display_name: "Impact factor",
level: 2,
score: 4.46396
}
// and so forth
]

counts_by_year

List: The values of works_count and cited_by_count for each of the last ten years, binned by year. To put it another way: for every listed year, you can see how many new works were tagged with this concept, and how many times any work tagged with this concept got cited.
Years with zero citations and zero works have been removed so you will need to add those back in if you need them.
counts_by_year: [
{
year: 2021,
works_count: 4211,
cited_by_count: 120939
},
{
year: 2020,
works_count: 4363,
cited_by_count: 119531
},
// and so forth
]
​

works_api_url

String: An URL that will get you a list of all the works tagged with this concept.
We express this as an API URL (instead of just listing the works themselves) because there might be millions of works tagged with this concept, and that's too many to fit here.
works_api_url: "https://api.openalex.org/works?filter=concept.id:C2778407487"

updated_date

String: The last time anything in this concept object changed, expressed as an ISO 8601 date string. This date is updated for any change at all, including increases in various counts.
updated_date: "2021-12-25T14:04:30.578837"

created_date

String: The date this Concept object was created in the OpenAlex dataset, expressed as an ISO 8601 date string.
created_date: "2017-08-08"

The DehydratedConcept object

The DehydratedConcept is stripped-down Concept object, with most of its properties removed to save weight. Its only remaining properties are:
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The Concept object
id
wikidata
display_name
level
description
works_count
cited_by_count
ids
image_url
image_thumbnail_url
international
ancestors
related_concepts
counts_by_year
works_api_url
updated_date
created_date
The DehydratedConcept object