"Threshold" comes from Old English (þrescold), denoting a doorsill or entry point.  It is a compound word: the first part relates to thresh (treading or stepping), and the second originally signified a piece of wood or base (from the same root as "hold"). 

According to the Oxford English Dictionary (OED), the noun threshold was first used in English during the Old English period (before 1150), with historical records showing examples like þeorscwold appearing around 888 AD. 

A threshold in storytelling marks the vital boundary where a character leaves the familiar safety of the ordinary world and steps into the unknown.  It drives the plot forward by forcing personal growth, creating structural tension, and establishing high stakes. 

Narrative Architecture and Progression: 
Commitment to the Quest: The exact moment a character steps past a physical or emotional line, signaling they are fully locked into their journey with no easy way back.  
World-Building Contrast: Separates the rules, values, and visual reality of the old life from the new, dangerous special world.  
Pacing Pivot: Shifts the story from setup and hesitation into active conflict, exploration, and trial. 

Character Transformation: 
Confronting Fear: Forces the protagonist to face deep-seated doubts and internal resistance before moving forward.  
The Point of No Return: Eliminates passive observation, requiring the character to adapt or fail under fresh pressures.  
Internal and External Mirroring: Uses physical gates, doors, or borders to reflect the painful or exciting psychological changes happening inside the hero. 

Just the facts:

THRESHOLD Definition & Meaning - Merriam-Webster

THRESHOLD Synonyms: 74 Similar and Opposite Words | Merriam-Webster Thesaurus

Word of the Day: Threshold | Merriam-Webster

ON/AT THE THRESHOLD Definition & Meaning - Merriam-Webster

THRESHOLDS Synonyms: 72 Similar and Opposite Words | Merriam-Webster Thesaurus