Stale price
A stale price is one that has not been updated to reflect current information and no longer represents what a venue would actually offer.
In plain English
Prices go stale when a feed lags, a book is slow to react, or a market is thin. Acting on a stale number is unreliable in both directions: it may not be available, and it may not be as good as it looks.
Example
A screen still shows -110 minutes after news that has moved every other book to -140.
Why it matters
A price you cannot actually get is not an opportunity, and a stale quote makes a record look better than reality.
How V8 uses this term
V8 records when observed pricing is too stale to support an official instruction, and reports that as a published decision reason rather than discarding the event silently.