Core terms
Low float
An informal label for a stock with a small float. There is no official threshold; traders often mean a few million to roughly twenty million shares.
With fewer shares available, a given amount of buying or selling can move the price further, so low-float stocks can make large percentage moves on news. The same thinness also means wide spreads and sudden drops when sellers appear.
The label depends on the float estimate, which can be stale. A company with a "low float" on a screener may have sold millions of new shares last week.
What traders should verify
Check the float's date and any offering, ATM or warrant activity since then. Size positions for the spread and the possibility of halts.
Do not conflate
Trader usage; no official threshold. A low float does not cause a stock to rise.
Primary source
Trader usage: there is no official definition, and platforms and tools may calculate it differently.