A bibliometric measure of obsolescence defined for a specified corpus and citation window: the elapsed time from a chosen origin until half of the corpus's total citations (within the window) have been accumulated. The measure quantifies how rapidly attention, as measured by citations, concentrates in earlier versus later periods for that corpus.

Principle

When applied to a defined corpus, the half‑life identifies the time interval that contains 50% of cumulative citations; shorter half‑lives indicate faster citation concentration toward recent works, longer half‑lives indicate slower obsolescence or longer persistence.

Demonstration

Illustrative scenario: For a journal's articles published in years 2000–2010, count citations received during 2000–2020 by publication year, compute cumulative citation share by age, and identify the age t at which cumulative share reaches 50%; t is the literature half‑life for that corpus and citation window.

Misapplication

Treating the half‑life of a corpus as the lifetime of individual items or assuming exponential decay of attention without testing; confusing half‑life computed on citations with measures based on downloads, readership, or expert judgment.

Consequence

Provides a quantitative basis for collection development, retention policy, and decisions about digitization or access prioritization by indicating typical currency of citations for the specified corpus and window.

Reversal

Fields with long citation latency, episodic ‘‘sleeping beauties,’’ or citation bursts that occur long after publication can yield misleadingly long or unstable half‑lives; measurement also changes with citation window selection and corpus definition.

Boundary

Clearly within: a defined journal or subject corpus with a specified citation window. Boundary case: interdisciplinary corpora where citations shift across fields. Clearly outside: using the term to describe individual article relevance or non‑citation usage metrics without recalculation.

Semantic Tension

Currency (recentness signaled by short half‑life) ↔ Historical value or long‑term impact (captured by long half‑life or post‑window citations).

Synthesis

The literature half‑life is a corpus‑level descriptor of citation timing that is useful for operational decisions only when its corpus and citation‑window assumptions are explicit and complemented by other impact or usage metrics.