|This article is written from the Real Life point of view|
A difficulty mode is a concept used in video games, including the Metroid Prime series, and all entries of the mainline series from Metroid Fusion on. In this sense, "difficulty" does not refer to how difficult a Metroid game is in general, but rather to a setting of the game chosen by the player at the beginning of their playthrough. Harder difficulties are often unlocked after the first playthrough has been completed, and generally have enemies deal more damage, although other challenges might be posed as well.
Main Metroid series
The Japanese version of Metroid Fusion features an Easy Mode and Hard Mode. The North American version, which was released first, does not; the Easy Mode was likely added in response to complaints about the difficulty. Metroid: Zero Mission is the first 2-D Metroid to feature multiple difficulties in the Western version of the game. In both games, Easy and Normal are available by default while Hard is unlocked by completing the game on Normal.
In both games, Hard Mode also affects expansion tanks, reducing the amount that they expand Samus's arsenal. Energy Tanks only provide 50 units of energy (half of their usual 100), Missile Tanks only expand Samus's Missile Gauge by 2, and Super Missile and Power Bomb Tanks only add one additional unit. In Zero Mission, harder difficulties have shorter time limits during the escape sequences.
In Metroid: Other M, Hard Mode is unlocked after achieving 100% completion. On top of increasing damage dealt by enemies, it removes all expansions in the game, essentially constituting a 0% run. This makes Samus especially vulnerable.
In Metroid: Samus Returns, a unique difficulty mode is unlockable via the Metroid amiibo: Fusion Mode, which changes Samus's Power Suit to the Fusion Suit. This difficulty level is significantly harder than Hard Mode, which is unlockable in the base game after completing it once, as it increases enemy damage by 4x what it is in Normal Mode.
Metroid: Samus Returns manual
The current game's difficulty level. * You can change the difficulty level for a file after completing the game and selecting "New Game".
Metroid Prime series
Metroid Prime is the first game to feature adjustable difficulty levels worldwide. Upon completion of the first playthrough on Normal Mode, Hard Mode is unlocked. Metroid Prime 2: Echoes and Metroid Prime: Federation Force continue this trend. Hard Mode also affects some of the Federation Force Campaign mission score bonuses, such as increasing the amount of points needed to be scored, or lessening the par-time.
Metroid Prime 3: Corruption introduces Veteran Mode, an "intermediate" difficulty, and replaces Hard Mode with Hypermode. New Play Control! Metroid Prime and Metroid Prime 2: Dark Echoes, as well as the Metroid Prime Trilogy, apply the Veteran and Hypermode difficulties to the first two games as well, although the former two still refer to it as Hard Mode. Hypermode is the same as the original Hard Mode, while Veteran is comparable to the Normal Mode of the original games, and an even easier Normal Mode is added. Unlike the 2-D games, harder difficulties in the Metroid Prime Trilogy will increase enemy health.
Metroid Prime Pinball features Expert Mode, which increases the difficulty of Mini-Games from level 1 to level 2 on the first play. Metroid Prime Hunters is the only Prime game to not have an adjustable difficulty setting.