Core terms
After hours
Extended-hours trading after the 16:00 ET close, usually until 20:00 ET on exchanges. After-hours trades do not change the official closing price.
Earnings releases, filings and offering announcements often come out after 16:00, so after-hours trading is where many moves start. Like premarket, the session usually has thinner liquidity and wider spreads than the regular session.
The official close comes from the listing exchange's closing auction. A stock can close at $5.00 and trade at $6.50 at 18:00; the next day's change is still measured from $5.00.
What traders should verify
When a filing hits after hours, note the time in ET and whether it is new information or a document about something already announced.
Do not conflate
Broker windows and order types differ. After-hours prices can reverse before the next open.