Short answers, pulled from the story.
005, released by Sega in 1981, holds the Guinness World Record for being the first stealth game. Manbiki Shounen, a type-in program published in November 1979 by Hiroshi Suzuki, is also cited as an early precursor by Retro Gamer's John Szczepaniak.
Tenchu: Stealth Assassins, Metal Gear Solid, and Thief: The Dark Project all released in 1998 and are credited with establishing the stealth game as a mainstream genre. Metal Gear Solid in particular is credited with popularizing the genre to a broad audience.
Ghosting refers to completing a stealth game's objectives without being detected by any enemy. It is a common approach in the genre, sometimes the only way to finish a game, and is also pursued voluntarily by players as a demonstration of skill even when detection is allowed.
Metal Gear Solid 2: Sons of Liberty, released in 2001 for the PlayStation 2, holds a Guinness World Record for being the first stealth game to feature collective artificial intelligence. It also sold 7 million units, making it the best-selling entry in the series at that time.
Thief: The Dark Project was the first stealth game to use the first-person perspective, dubbed a "first-person sneaker." It introduced a sound design system where players could track unseen enemies by ear, and established that movement across different surfaces, such as stone versus carpet, should produce different noise levels.
Dishonored, released in 2012, gave players several options when detected, including attacking, distracting enemies, or fleeing via parkour, rather than triggering automatic mission failure. Splinter Cell designer Clint Hocking had acknowledged that the harsh detection penalty in early Splinter Cell entries would remain a frustration for the genre for about a decade.