Core terms
Gapper
Trader shorthand for a stock that opens, or is trading before the open, well above or below the prior close. Lists of the day's gappers are a common starting point for small-cap day traders.
"Gapper" usually means a gap up, often 10% to 20% or more, on news or a filing. There is no official threshold, and lists differ on the cut-off, the minimum volume and whether they use the open or the latest premarket price.
FennEdge's Top Gappers list uses US common stocks and ADRs with at least 1 million shares of volume, ranked by opening gap. In FennEdge's study of 671 stocks that opened at least 20% above the prior close (2026-07-01 to 2026-10-02; as of 2026-10-02), 65.3% closed below their open.
What traders should verify
Before trading a gapper, know why it gapped, its float and dollar volume, and whether the company has offering capacity.
Do not conflate
Trader usage; no official definition. Being on a gapper list says a stock moved, not that it will keep moving.
Primary source
Trader usage: there is no official definition, and platforms and tools may calculate it differently.