A map made by someone other than Bungie. [a reader informed me that a 1st party map would be a map created by bungie for internal use, a 2nd party map is one created by bungie for external use, and a 3rd party map is one created by someone outside of bungie. They compared it to "voices" in literature but I don't think it really stands up. Here's what I think.
1st party map: A map made by Bungie and released as part of the official Game release.
2nd party map: A map made by Bungie employee(s) or with the help of Bungie employees and released by Bungie after the original game was released. (ex. Chimera, Leagues From Nowhere, Cryptic Wightings, Going to Town, and some other maps like some of Badlands maps (made by bungie employees on their own time), or the Pepper Heights map for "Myth III : The wolf age" which shipped with copies of Myth III sold at Electronic Boutique stores)
3rd party map: A map made and distributed by someone other than Bungie.