In the world of NFT art, an open edition is an NFT for which any number of editions can be minted. This contrasts with a limited edition NFT: one that is limited to a predefined number of editions.
Open editions are generally seen as less rare or prestigious than limited editions, since they are structured to satisfy demand for the piece in the primary market. Edition limits, by contrast, imply that fewer editions will be produced than there is demand for - ensuring that prices for the piece stay high due to competition for ownership.
However, open editions can still become quite valuable in circumstances such as the following:
The artist gains popularity after the open edition release, creating stronger demand for their earlier works.
If only a few open editions are minted, those pieces become rare by definition.
The artist adds value to the NFT, such as by airdropping perks to holders, or making them eligible for other exclusive rewards and amenities.
The mechanics of open editions can vary. On some curated NFT art marketplaces, such as Nifty Gateway, open editions are still limited-run productions - they are just not limited by a specific number of editions. On that platform, buyers only have a small window of time in which an open edition piece is available to buy. There will be as many pieces minted as are bought in that window of time. Once the window closes, no more editions of that NFT will be minted.
Open editions can be a type of drop, and therefore a marketing tool. For most NFTs created on non-curated platforms like OpenSea and Rarible, though, it's not necessary to specify whether an NFT is part of a limited or open edition if the artist is not in high demand.
Open editions may or may not have an edition number, and may or may not advertise the total number of editions created.
In early 2021, the artist toomuchlag dropped a collection called "My Journey" on Nifty Gateway that featured both normal (limited) editions and open editions. The four NFTs in the main (limited) collection were sold at auction and limited to 25, 15, 3, and 1 edition, respectively. This means that each of the four pieces would produce only that many editions (as long as there were enough buyers in the auction to fill up that number).
The open edition collection featured a single NFT called Le Anime that had no limit on the number of editions that could be purchased. In the few minutes it was available to buy, 1,572 editions were purchased. Therefore, 1,572 editions of Le Anime were minted.