Definition

Open access

Publishing that makes research freely readable — gold (journal-side, often via APCs), green (self-archived copies), diamond (free for both sides).

The colors encode who pays and where the copy lives: gold means the journal makes the version of record free (often funded by article processing charges); green means the author archives a version in a repository; diamond journals charge neither readers nor authors; hybrid journals sell OA per-article inside subscriptions.

Funder mandates (Plan S, national policies) increasingly require OA, and openly available papers are the raw material of tools built on open scholarly data. For finding legal open copies of paywalled papers, repository versions are the standard route.