Plain-language definitions of the identifiers, formats, and citation terms researchers use every day.
Digital Object Identifier. A permanent link to a published work - articles, preprints, datasets, and book chapters. A DOI always starts with “10.” followed by a registrant code, a slash, and a suffix (e.g., 10.1038/s41586-023-05881-4). Unlike URLs, DOIs do not break when a publisher moves their website.
Scholar Sidekick tool: DOI Lookup · Authoritative source: doi.org
PubMed ID. A numeric identifier assigned by the US National Library of Medicine to every record in PubMed, the world’s largest biomedical citation database. PMIDs are typically 7–9 digits.
Scholar Sidekick tool: PubMed ID Converter · Authoritative source: PubMed
PubMed Central ID. Identifies full-text open-access articles in PubMed Central. Always starts with “PMC” followed by digits (e.g., PMC3737249). A paper can have a PMID without a PMCID (when the full text is not open-access) or both.
Scholar Sidekick tool: PubMed ID Converter · Authoritative source: PubMed Central
International Standard Book Number. Identifies books, monographs, and book-like publications. ISBN-10 has ten digits; ISBN-13 has thirteen. ISBN-13 is current; both forms remain in use.
Scholar Sidekick tool: DOI Lookup (accepts ISBNs) · Authoritative source: isbn-international.org
International Standard Serial Number. An eight-digit identifier for journals and serial publications (e.g., 0028-0836 for Nature). A journal can have separate print and electronic ISSNs; the electronic form is sometimes called an eISSN.
Scholar Sidekick tool: Scholarly Identifier Extractor · Authoritative source: issn.org
Identifier for preprints on the arXiv repository. The modern format is YYMM.NNNNN (e.g., 2305.12345) with an optional version suffix (v1, v2). Pre-2007 papers use a category prefix (e.g., physics/0301001).
Scholar Sidekick tool: DOI Lookup (accepts arXiv IDs) · Authoritative source: arxiv.org
A nineteen-character identifier used by NASA’s Astrophysics Data System. Encodes year, journal code, volume, page, and first-author initial (e.g., 2023Natur.615..123W). Common in astronomy and astrophysics literature.
Scholar Sidekick tool: Scholarly Identifier Extractor · Authoritative source: NASA ADS
A link to a document in the World Health Organization’s Institutional Repository for Information Sharing (IRIS). Used for WHO reports, guidelines, and grey literature that do not have a DOI.
Scholar Sidekick tool: DOI Lookup · Authoritative source: iris.who.int
A plain-text reference file format used alongside LaTeX. A .bib file contains one or more entries (@article, @book, @inproceedings, etc.) describing publications. BibTeX is the lingua franca of STEM bibliographies and is widely supported by reference managers.
Scholar Sidekick tool: DOI to BibTeX
Research Information Systems format. A tagged text format (TY, AU, TI, JO, PY, etc.) used to move references between reference managers — Zotero, Mendeley, EndNote, RefWorks, JabRef. A single .ris file can contain many records.
Scholar Sidekick tool: DOI to RIS
Two reference-interchange formats supported by the EndNote reference manager. The older “Refer” form uses plain-text tags (%A, %T, %J); the newer EndNote XML is structured. Both are accepted by most competing reference managers.
Available via: Scholar Sidekick export API
A JSON representation of bibliographic metadata defined by the Citation Style Language project. It is the canonical intermediate format a CSL engine consumes to render a citation in any style.
Available via: Scholar Sidekick export API
A set of rules governing how bibliographic references are formatted. Different journals, disciplines, and institutions require different styles. The differences are not only cosmetic: author ordering, name abbreviation, title casing, journal abbreviation, and DOI formatting all vary.
Citation systems fall into two broad families: numbered styles (Vancouver, IEEE, AMA) where references appear in order of citation in a numbered list, and author–date styles (APA, Chicago, Harvard) where in-text citations carry the author’s surname and year. Most journals specify a required style in their submission guidelines and sometimes layer their own house rules on top.
Scholar Sidekick tool: Citation Style Comparator
An XML-based language for describing citation formats. The CSL project maintains a community-curated repository of 10,000+ styles, including journal-specific formats (Nature, The Lancet, Cell, JAMA) and generic styles (APA, Chicago, MLA). Most reference managers use CSL to render citations.
CSL separates bibliographic data (CSL JSON) from formatting rules (CSL XML styles), so a single set of metadata can be rendered into any supported style. Originally developed by Bruce D’Arcus, CSL is now the de facto standard: Zotero, Mendeley, Papers, Pandoc, and many academic writing tools render their citations through a CSL engine.
Scholar Sidekick tool: Citation Style Comparator · Authoritative source: citationstyles.org
A publishing model in which research articles are freely available to read, reuse, and redistribute. Commonly classified as Gold (published OA), Green (author-archived), Hybrid (OA within a subscription journal), Bronze (free to read but no licence), or Diamond (free to read and publish).
Open access adoption accelerated after funder mandates from agencies including the US NIH, UK Research and Innovation, and cOAlition S’s Plan S, which require publicly funded research to be freely accessible. Gold OA is typically paid for through article processing charges (APCs); Green OA relies on self-archived versions in institutional or disciplinary repositories such as arXiv, bioRxiv, and PubMed Central. Embargo periods sometimes delay Green OA availability by six to twelve months after publication.
Scholar Sidekick tool: Open Access Checker · Authoritative source: Unpaywall
Formal withdrawal of a published paper, typically due to errors, misconduct, or ethical violations. A retracted paper remains visible in databases but is marked as retracted. Related statuses include expressions of concern and corrigenda.
Retractions are issued by the journal editor after investigation, guided by the Committee on Publication Ethics (COPE) retraction guidelines. Reasons include honest error, data fabrication, image manipulation, undisclosed conflicts of interest, and duplicate publication. The volume of retractions has grown as post-publication scrutiny has increased, but the process from first concern to formal retraction can still take years — meaning a paper’s retraction status is worth checking even after initial citation.
Scholar Sidekick tool: Retraction Checker · Authoritative source: Retraction Watch
Checking that a claimed citation actually corresponds to a real paper. The word hides two different guarantees: that the cited work exists (a title search succeeds) and that the identifier you will publish resolves to it (a registry lookup succeeds). A citation can pass the first and fail the second — for example, a real paper cited under the wrong DOI — so it is worth knowing which question a tool answered.
Verification is distinct from a retraction check (which asks whether a real paper was withdrawn) and from claim-support checking (which asks whether the cited paper actually backs the statement it is attached to). Scholar Sidekick’s verifier reports four outcomes: matched, mismatch, ambiguous, and not found.
Scholar Sidekick tool: Citation Verifier
Taking the identifier you supplied — a DOI, PMID, PMCID, arXiv ID, ISBN, ISSN, ADS bibcode, or WHO IRIS URL — and looking it up against its authoritative registry to retrieve the real record it points to. Resolution answers identity: does this exact link lead to the work you say it does. It is stronger than title matching, which only answers whether a paper with the claimed title exists.
Scholar Sidekick tool: Citation Verifier
Confirming a citation by searching a corpus or the web for a paper whose title (and perhaps authors and year) looks like the claim. It is fast and needs no identifier, but it answers existence rather than identity, so it is blind to which identifier you will actually publish — a real title paired with the wrong DOI can pass a title-matching check.
Scholar Sidekick tool: Citation Verifier
A citation that stitches a real, resolvable identifier to an invented title — the dominant AI-driven fabrication pattern documented by Topaz et al. (Lancet 2026) across 2.5 million biomedical papers. Because the identifier is live, a naive check that only confirms the DOI exists passes it; catching it requires comparing the claimed title against the title the identifier actually resolves to.
Scholar Sidekick tool: Citation Verifier · Authoritative source: Topaz et al. (Lancet 2026)