A deckle edge is a rough, uneven page edge. Originally it was the natural feathered edge of handmade paper; today it is often imitated by machine on literary hardbacks and fine editions to give a traditional look.
How to spot it
Look at the fore-edge of the pages, the side opposite the spine. Deckled pages end unevenly, each sheet slightly different, instead of being trimmed flush. On modern books it is usually only the fore-edge; on handmade paper it can be every edge. Some listings call it "rough cut".
What it does to value
A deckle edge is a feature, not a fault, and it can help identify an edition: some first printings have deckled fore-edges that later printings lack, or the reverse. Buyers occasionally mistake deckle edges for damage, so dealers mention them to avoid returns. Torn or chipped deckles do count as wear.
See book condition grading for how edges are described, and points of issue explained for how physical details identify a printing.