Zork is a text adventure game first released in 1977 by developers Tim Anderson, Marc Blank, Bruce Daniels, and Dave Lebling for the PDP-10 mainframe computer. The original developers and others, as the company Infocom, expanded and split the game into three titles—Zork I: The Great Underground Empire, Zork II: The Wizard of Frobozz, and Zork III: The Dungeon Master—which were released commercially for a range of personal computers beginning in 1980. In Zork, the player explores the abandoned Great Underground Empire in search of treasure. The player moves between the game's hundreds of locations and interacts with objects by typing commands in natural language that the game interprets. The program acts as a narrator, describing the player's location and the results of the player's commands. It has been described as the most famous piece of interactive fiction.

The original game, developed between 1977 and 1979 at the Massachusetts Institute of Technology (MIT), was inspired by Colossal Cave Adventure (1976), the first well-known example of interactive fiction and the first well-known adventure game. The developers wanted to make a similar game that was able to understand more complicated sentences than Adventure's two-word commands. In 1979, they founded Infocom with several other colleagues at the MIT computer center. Blank and Joel Berez created a way to run a smaller portion of Zork on several brands of microcomputer, letting them commercialize the game as Infocom's first products. The first episode was published by Personal Software in 1980, after which Infocom purchased back the rights and self-published all three episodes beginning in late 1981.

Zork was a massive success for Infocom, with sales increasing for years as the market for personal computers expanded. The first episode sold more than 38,000 copies in 1982, and around 150,000 copies in 1984. Collectively, the three episodes sold more than 680,000 copies through 1986, comprising more than one-third of Infocom's sales in this period. Infocom was purchased by Activision in 1986, leading to new Zork games beginning in 1987, as well as a series of books. Reviews of the episodes were very positive, with several reviewers calling Zork the best adventure game to date. Critics regard it as one of the greatest video games. Later historians have noted the game as foundational to the adventure game genre, as well as influencing the MUD and massively multiplayer online role-playing game genres. In 2007, Zork was included in the game canon by the Library of Congress as one of the ten most important video games in history.

Zork is a text-based adventure game wherein the player explores the ruins of the Great Underground Empire. The player types text commands for their character to traverse locations, solve puzzles, and collect treasure. The game has hundreds of locations, each with a name and description, and the player's commands interact with the objects, obstacles, and creatures within them. Commands can be one or two words (e.g., "get lamp" or "north") or more complex phrases (e.g., "put the lamp and sword in the case"). The command must fit the location's context (e.g., "get lamp" works only if a lamp is present). The program acts as a narrator, describing to the player their location and the results of certain actions. If the game does not understand the player's commands, it asks for the player to retype their actions. The program's replies are typically in a sarcastic, conversational tone, much as a Game Master would use in leading players in a tabletop role-playing game.

The original 1977 version of the game was a single release, Zork. When it was converted into a commercial software title, it was divided into three episodes, with new and expanded sections added to the latter two episodes. Much of the game world is composed of puzzles that must eventually be solved, such as a set of buttons on a dam or a maze to be traversed. Some puzzles have more than one solution. For instance, since the "Loud Room" is too overwhelmingly loud for the player to perform actions, the player can either empty the nearby dam to stop the sound of water falling, or shout "echo" in the room to change its acoustics. In the first episode, or Zork I, a thief character is wandering the underground as well, taking items that have been left behind or even stealing from the player's possessions. The player can fight or evade the thief, and can recover stolen items from the thief's treasure room. Some locations contain antagonists that the player must fight or overcome. Beginning in Zork II the player can learn magic spells to use in puzzles and combat. In dark areas, the player must carry a lantern or other light source to avoid being eaten by a monster called a grue. There is a limit to how much "inventory" one can carry, determined by the combined weight of objects, rather than the quantity.

A principal goal of each episode is to collect all the treasures, many of which are hidden behind puzzles. As treasures are collected or tasks are accomplished, the player's score increases, providing a rough measure of how much of the game has been completed. The player may traverse the game world and solve puzzles in almost any order, although some passageways require problem-solving to get through, and some puzzles require the player to possess something gained from solving a different puzzle. In Zork III, unlike in prior episodes, there is a timed component that directly affects the outcome. An earthquake will occur after about 130 moves, opening one passageway and closing another. In each episode, the treasures are needed to reach the conclusion of the game.

Zork does not follow a linear storyline. Most of the setting is established through the game's written descriptions of items and locations, as well as manuals in later game releases. Long before the time the game is set in, the Quendor empire, having conquered everywhere above ground, built a massive cave complex to expand. Two hundred years later, the ruler Lord Dimwit Flathead renamed the empire to the Great Underground Empire and spent his reign building massive, largely pointless projects such as an underground dam and the royal museum. A century later, the empire's overspending caused it to collapse, and all the residents left. The abandoned empire is the setting of the three episodes of Zork.

Zork I begins with the unnamed player standing in an open field west of a white house, with a boarded front door; most of the game occurs underground, as do the subsequent episodes. In Zork II the player learns of the Flatheads, and meets the Wizard of Frobozz, who was once a respected enchanter but was exiled by Lord Dimwit Flathead when his powers began to fade. The wizard appears randomly throughout the game and casts spells that begin with the letter "F" on the player. These have several effects, such as "Fluoresce", which causes the player to glow, and "Freeze", which keeps the player stuck in place for a few turns. In Zork III the player character gathers the garb of the Dungeon Master to become his successor. Once the player has all the items, they must feed an elderly man, who reveals himself as the Dungeon Master and shows them the doorway leading to the final hallway. After the player solves the final puzzles, the Dungeon Master appears and transforms the player to look like himself, signifying the player's succession to his position.