The word unity comes frm the Latin unitas (oneness), derived from unus (one).  It entered the English language via Old French and Anglo-Norman (unité), replacing the older Old English word ānnes.  

According to the Oxford English Dictionary, the noun unity was first used in English around 1330 during the Middle English period.  Originally it was used in English texts to describe the state or property of being one, as documented in early Middle English literature such as the Speculum Guy of Warwick.  In the late 14th-century its meaning expanded into more general usage as meaning an indivisible or harmonious combination of parts. The OED records the word unite as first appeared in English during the Middle English period.   The earliest recorded form is as an adjective around 1429 in Mirour of Mans Saluacioune, and the verb form appears shortly after, recorded before 1475 in R. Higden's Polychronicon.  The noun form didn't appear until 1604 in the Proclamation on Coins, referring to a gold coin. 

Unity in storytelling means that every part of a narrative--plot, characters, setting, and theme--works together to create a single, focused impact.  Just like Edgar Allan Poe’s concept of "unity of effect," a unified story avoids random subplots or mixed messages, ensuring the audience stays engaged and deeply understands the core message.  

Why Unity Matters:  
Clear Focus: Keeps the plot moving forward without confusing side tracks.  
Stronger Emotion: Makes the ending feel earned and powerful because all actions lead to it.  
Thematic Depth: Connects character growth directly to the main message of the story.  
Better Memorability: Helps the audience remember and relate to the core experience.  

How to Achieve Unity: 
Connect the Plot: Ensure every event stems from a clear character choice or central conflict.  |
Align the Theme: Make sure secondary characters and subplots reflect the main idea.  
Refine the Ending: Know your destination so every early scene supports the final payoff.