The word "revelation" literally translates to an "unveiling" or "disclosing".   It traces back to the late 14th-century Middle English revelacioun, rooted in Old French and the Latin revēlātiō, noun of action from past-participle stem of revelare "unveil, uncover, lay bare" (see reveal).  It is also the standard translation of the Greek term apokalyptein (found in the biblical Book of Revelation/Apocalypse), which combines apo ("off, away from") and kalyptein ("to cover, conceal").  

According to the Oxford English Dictionary (OED), the noun revelation was first used in English during the Middle English period (1150-1500), with the earliest recorded evidence appearing around 1384 in the early version of the Wycliffite Bible. 

In storytelling, revelation is the moment a hidden truth is disclosed to characters or the audience. It is vital because it drives character arc, shifts the narrative's direction, and transforms the audience's understanding , turning passive observation into an active, emotional realization.  

Just the facts:

REVELATION Definition & Meaning - Merriam-Webster

REVELATION Synonyms: 16 Similar and Opposite Words | Merriam-Webster Thesaurus

REVELATIONS Definition & Meaning - Merriam-Webster