The Web Content Accessibility Guidelines (WCAG) are the W3C's recommendations for making web content accessible. They organise success criteria under four principles -- Perceivable, Operable, Understandable, Robust ("POUR") -- and assign each a conformance level: A (essential), AA (industry standard), or AAA (enhanced).
WCAG 2.2 (October 2023) is the current version, adding nine criteria over WCAG 2.1. WCAG 3.0 is in progress but not yet a recommendation. Most regulatory frameworks (US Section 508, EU EN 301 549, UK PSBAR, Ontario AODA) reference WCAG 2.1 AA or 2.2 AA as the legal baseline.
In practice, "accessible" means meeting WCAG 2.2 AA. Common AA requirements: 4.5:1 contrast for body text, keyboard navigation for every interactive element, visible focus indicators, alt text on every meaningful image, and descriptive page titles.