Form 8-K
Also called 8-K, current report.
The “current report” a public company files to announce a major event, usually within four business days.
Companies file 8-Ks as events happen, instead of waiting for the next quarterly or annual report. Most items are due within four business days of the event. Source 1 Source 2
Each kind of event has an item number, such as 2.02 for earnings results or 4.01 for a change of auditor. See What the 8-K items mean. Source 2