Sprint

Were you looking for the video series The Sprint? "Feel the need? Engage this Ability for a burst of speed that's great for quickly covering ground or escaping sticky situations."

- Bungie

Sprint is a gameplay mechanic that enables the player to move faster than usual. When active the player can run twice as fast as walking speed, but cannot use their weapons. Sprint can be used for evasion, flanking, or speedrunning, and has been in every Halo FPS after Halo: Reach.

Halo: Reach
In Halo: Reach, Sprint is an armor ability activated by the Sprint armor plug. According to Colonel Holland, Kat-B320 invented the device as a "hack" that briefly overrides the MJOLNIR suit's safety regulators so that the user can move faster than usual for a limited time. Since overuse could result in the suit overheating, the Sprint plug deactivates after 5 seconds, then has 2.5 seconds of cooldown before it can be used again.

Halo 4
In Halo 4, Sprint is an inherent trait and no longer an armor ability. This allows the player to use it in conjunction with other armor abilities and not have to swap it out. However, it still has a timed period of use and cooldown, which are not visible onscreen unlike armor abilities.

Halo 5: Guardians
In Halo 5: Guardians, Sprint is still inherent, though now considered a Spartan Ability. Unlike previous games, it has no cooldown and so can be used infinitely. However, in multiplayer sprinting will delay shields recharging, requiring the player to stop or walk to recover from damage. Sprint is necessary for use of other Spartan abilities like Spartan Charge, using its running start to build up strength for a smash attack.

Production note
At one time, Sprint was planned to be in Halo 2. Sprint would've been automatic in that version, immediately increasing the player's speed if they kept walking forward for a certain distance. However it was cut early in development, leaving only a few scrapped animations in the game's engine. Curiously, the animation for sprinting while dual wielding SMGs depicted the player raising both their middle fingers.