Core terms
Iceberg (reserve) order
A limit order that shows only part of its size on the book and refills the displayed part from hidden size as it trades. Exchanges call it a reserve order.
Nasdaq defines the feature as Reserve Size (Rule 4703(h)); NYSE defines a Reserve Order (Rule 7.31(d)(1)). When the displayed portion trades, it is replenished from the reserve. On Nasdaq the refreshed displayed portion receives a new timestamp, so it joins the back of the displayed queue at that price (rule text as of 2026-10-05).
Non-displayed orders
A non-displayed limit order shows no price or size at all: on Nasdaq the Display attribute is switched off (Rule 4703(k)); NYSE covers it in Rule 7.31(d)(2). At the same price, displayed orders trade before non-displayed ones, so hiding trades queue position for discretion. Midpoint peg orders also rest unseen and can execute at sub-penny prices.
On a retail screen, hidden size leaves ambiguous clues: more shares printing at a price than were ever displayed there, or displayed size returning to the same number after each trade. Several traders joining one price, a market maker refreshing a quote, or venues missing from the feed produce the same picture, and no clue shows how much size remains.
What traders should verify
Log the observation ("prints exceeded display at the level") rather than a conclusion ("iceberg at the level"), and record how each logged level ended: held, broke or was pulled.
Do not conflate
The order types are defined in exchange rules, but identifying one from Level 2 and Time & Sales is inference. The same refill pattern appears before a level holds and before it breaks.
Primary source
Primary sourceNasdaq Equity 4, Rule 4703(h) and (k)