Single-Segment and Segmented
Single-Segment runs and Segmented runs refer to two different types of speedruns.
Single-Segment Speedruns
Single-Segment Speedruns refer to speedruns that require playing through a Prime game from the defined start of the run until the defined end without resetting or dying. These runs are typically required to be performed in a single sitting. The reasoning for disallowing resetting and dying is due to the in-game time only counting time that was up to the last save point after resetting to it, meaning that a runner could continually reset to their last save if they were not satisfied with their time.
Segments and Segmented Speedruns
Segments refer to parts of a speedrun, typically distinguished by the time between exiting a Save Station and entering one. Because in-game time is paused during save station cutscenes, runners began performing Segmented Speedruns, where a runner would start a run and try to get the fastest possible time to the first save station or an early save station. The runner may reset continuously until achieving a satisfactory time and then "locking in" their Segment by saving at a save station, then moving onto the next Segment of the speedrun and repeating this with multiple save stations until reaching the end of the game.
A good example of a category that runners do Segmented Speedruns of are of Metroid Prime's Low% category, also known more commonly as 21%, because of its difficulty and how easy it is to die.