4X games are a genre of strategy video game in which players control an empire and "explore, expand, exploit, and exterminate". The term was first coined by Alan Emrich in his September 1993 preview of Master of Orion for Computer Gaming World. Since then, others have adopted the term to describe games of similar scope and design.
4X games are noted for their deep, complex gameplay. Emphasis is placed upon economic and technological development, as well as a range of non-military routes to supremacy. Games can take a long time to complete since the amount of micromanagement needed to sustain an empire scales as the empire grows. 4X games are sometimes criticized for becoming tedious for these reasons, and several games have attempted to address these concerns by limiting micromanagement.
The earliest 4X games borrowed ideas from board games and 1970s text-based computer games. The first 4X games were turn-based, but real-time 4X games are not uncommon. Many 4X games were published in the mid-1990s, but were later outsold by other types of strategy games. Sid Meier's Civilization is an important example from this formative era, and popularized the level of detail that would later become a staple of the genre. In the new millennium, several 4X releases have become critically and commercially successful.
Difficulties in definition
While many strategy games arguably contain a similar "explore, expand, exploit, exterminate" cycle, game journalists, developers and enthusiasts generally apply "4X" to a more specific class of games, and contrast 4X games with other strategy games such as Command & Conquer. Hence, writers have tried to show how 4X games are defined by more than just having each of the four Xs. Gaming sites have stated that 4X games are distinguished by their greater complexity and scale, and their intricate use of diplomacy beyond the standard "friend or foe" seen in other strategy games. Reviewers have also stated that 4X games feature a range of diplomatic options, and that they are well known for their large detailed empires and complex gameplay. In particular, 4X games offer detailed control over an empire's economy, while other strategy games simplify this in favor of combat-focused gameplay.
Source: Wikipedia, "4X game", available under the CC-BY-SA License.
Do take note of that second part. Not all 4 Xs need to be present, maybe a better description would be expand, exploit, exterminate, extremely large learning curve - compared to most videogames. Also mind Wikipedia's remarks about great control over economy and diplomacy.
Examples of 4X:
Space Empires V
Command & Conquer (RTS, not 4X)