Short answers, pulled from the story.
A random encounter is a feature in role-playing games where combat or other events with non-player character enemies or dangers occur sporadically and without the enemy being physically visible beforehand. The mechanic simulates the hazards of dangerous environments such as monster-infested dungeons or wilderness areas. It differs from a placed encounter, which is scripted and appears at a fixed location.
Random encounters originated in Dungeons and Dragons in the 1970s, where they were sometimes called wandering monsters. A gamemaster rolled dice against a random encounter table to determine whether an encounter occurred and what creatures appeared. The mechanic carried over into video games by the early 1980s, appearing in Wizardry in 1981.
Dragon Quest, Final Fantasy, and The Bard's Tale were genre-defining games of the mid-1980s that relied heavily on random encounters. Pokemon and the Final Fantasy series are among the most-cited examples of Japanese role-playing games with frequent random encounters. The early Dragon Quest games were particularly noted for allowing encounters to occur one step after another.
Random encounters use pseudo-random number generators to determine whether a fight triggers with each step the player takes. A more sophisticated algorithm sets a counter to a random integer between 64 and 255 and decrements it by varying amounts depending on terrain, triggering a fight only when the counter drops below zero. This guarantees at least eight steps between any two encounters regardless of terrain type.
Players increasingly complained that random encounters were annoying, repetitive, and discouraging to exploration. The Final Fantasy and Tales series abandoned random encounter systems in later games, and newer franchises like the Chrono series and Kingdom Hearts never adopted the mechanic. Developers moved toward finite enemy counts or visible spawning systems that give players more agency.
Two main systems replaced random encounters. The first uses a finite number of enemies per area, as seen in Final Fantasy XII, Radiata Stories, Fallout, and Fallout 2. The second uses visible spawning, where enemies reappear at fixed, predictable locations, as in Chrono Trigger and most of Dragon Quest IX. Both approaches allow players to anticipate, evade, or select encounters rather than having them arrive unexpectedly.