Core terms
Low of day (LOD)
The lowest trade price so far in the trading day, with the same session caveat as high of day: say whether premarket and after-hours trades count.
The low of day is the floor of the day's range so far. On a gap-up stock it is often set early, before the open or in the first minutes, and later prices are compared with it. On a fading stock a "new low of day" late in the session means every earlier buyer at that price or above is now under water.
A stock that gaps up can still have a low of day far above the prior close, because the whole day traded above it. That is why "red on the day" (below the prior close) and "at the low of day" are different statements.
What traders should verify
Mark the regular-session low separately from the premarket low. When a stop sits just under the low of day, expect other traders' stops there too.
Do not conflate
Trader usage; no official definition. As with high of day, tools differ on session and trade-condition filters.
Primary source
Trader usage: there is no official definition, and platforms and tools may calculate it differently.