Gradius is a series of shooter video games, introduced in 1985, developed and published by Konami for a variety of portable, console and arcade platforms. In many games in the series, the player controls a ship known as the Vic Viper. (via Wikipedia)
The genre Gradius helped define
Horizontal-scrolling shooter. a power-up bar the player spends deliberately, which made loadout choice the genre's strategy layer.
The game mechanics of Gradius
Scrolling MovementTraversalWorld scrolls in one or two axes as the player moves or time passes; player exists within an advancing boundary.
Boss EncounterCombatA single high-health enemy with unique scripted behavior and distinct phases serves as a gated progress challenge.
Contact DamageCombatTouching an enemy or environmental hazard deals damage to the player without any explicit attack trigger from either party.