A server emulator is the reimplementation of MMORPG game servers, typically as clones of proprietary commercial software by a third party. The term private server is also used while it may not be accurate as often the legitimate server is also privately owned. The private server is not always made by the original company, but usually attempts to mimic it in some way.
Technically, a server emulator does not emulate by the traditional definition. Instead it is the alternative implementation of the proprietary gaming server that communicates with the same gaming client through the same, reverse-engineered proprietary protocols. Server emulators exist for many online games.
The term server emulator also has other uses. For instance, VMware Server is sometimes called a "server emulator" as it emulates hardware for the server to run.
Unlike in cases of phishing, the emulator usually does not try to steal identity of the legitimate server. Original server software that is stolen, like AEGIS, is also not a server emulator. Reimplementations of standardized protocols or server behavior is not considered to be emulation.
According to a study based on Ragnarok Online emulated servers, “Players turn to the illegal private server solution to fulfill their expectations for better means of avatar customization, specific technical features, an improved social environment, and enhanced gamemaster availability.”  Other reasons why players use server emulators is to avoid the monthly fees or purchasing fees for certain games, as well as to experience an accelerated and/or altered mode of play. Some server emulators are designed on the emphasis that a game has closed, like Toontown Online.
Emulating the server of the proprietary commercial game often violates EULA as many commercial MMORPGs require the user to sign a clause not to create or use server emulators. It may also violate DMCA and has all issues, typical for reverse engineering of any kind. The server may try to avoid violations by serving from the country where some intellectual property laws apply differently or not at all.
Another issue is a possible infringement of the game creator's copyright. If the complete emulator is a work of its own, copyright violation is not as obvious as EULA violation (see Lotus v. Borland case). However sometimes the original server software leaks out of the company that created the game, for example AEGIS (Ragnarok Online). Use or distribution of leaked code is widely held to be copyright infringement. There are cases where a game creator has effectively shut down private game servers by threatening lawsuits due to intellectual property violations, such as offering a modified client (see information on NEXON v OdinMS) for download or offering downloads of modified files from the original game package.
In July 2011, Nexon has threatened to take MMORPG development community RaGEZONE to court over users creating and sharing custom emulated servers. Nexon claims to file legal proceedings against all parties involved in the MMORPG development scene. Disney has also fought against server emulators for it's MMO Club Penguin, resulting in the closure of iCPv3 in October 2010, which had over 100,000 users when Disney filed a cease and desist notice against the emulator.
In August 2011, a California Central District Court awarded Blizzard Entertainment $83 million in a lawsuit against Scapegaming over copyright-infringement. Scapegaming’s violation involved operation of an unauthorized copyrighted version of World of Warcraft. Scapegaming ran microtransactions encouraging players to donate money to advance in the game resulting in $3,053,339 of inappropriate profits. This is one of the first big cases implemented against server emulation.
Game Server Emulator List
-  - Announcement of a Star Wars Galaxies server emulator on slashdot.
-  - RaGEZONE announcement on Nexon and legal proceedings.
- Debeauvais, T.; Nardi, B. (2010). "A qualitative study of Ragnarok Online private servers: in-game sociological issues.". In Proceedings of the Fifth International Conference on the Foundations of Digital Games (FDG’10).
- Victoria Slind-Flor (2 Aug 2005). "DreamWorks, Blizzard, SABC, Utz, FirstRand, WTO: Intellectual Property". Bloomberg. Retrieved 2011-12-01.