Core terms
Premarket high
The highest trade price before the 09:30 ET open on the current day. Day traders use it as a reference level once the regular session starts.
The premarket high summarizes how far buyers pushed the price on thin extended-hours volume. After the open, traders watch whether the regular session trades above it.
The level is fragile. Premarket volume is a fraction of regular-session volume, a few small trades can set the high, and broker data feeds differ on which extended-hours prints they show. In FennEdge's study of 671 gap-ups of 20% or more (2026-07-01 to 2026-10-02; as of 2026-10-02), 315 (46.9%) did not set their extended-hours high in premarket, which means a later print traded above the premarket high.
What traders should verify
Note the time the premarket high was set and the volume behind it. A high set at 04:05 on a few hundred shares is a different reference from one set at 09:20 on heavy volume.
Do not conflate
Trader usage; no official definition. Whether a premarket high is "broken" depends on the data feed and on whether odd-lot trades are shown.
Primary source
Trader usage: there is no official definition, and platforms and tools may calculate it differently.