This word is primarily used with great imprecision by inarticulate, pretentious college undergraduates who wish to express no more than "a lot," but want to do it with as many syllables and as annoyingly as possible.
Unbearable undergraduate#1: I just drank a plethora of coffee so I can study for the test today.
Unbearable Undergraduate#2: Dude, me too. I hope there's not a plethora of questions.
It used to refer to a very specific type of poem, like The Iliad or The Odyssey. Then came the movie version of Lord of the Rings, arguably an epic itself. The movie's popularity introduced the word to the ignorant public. Soon hack video game "journalists" were using the term as a synonym for "fantasy," which did not help preserve the original definition. After years of corruption, the word is now an even more annoying synonym for "totally cool, dude!"
A tired metaphor for the different tasks a single person in a particular occupation is responsible for performing, primarily employed by stupid, annoying, pretentious people. Avoid these people if you can.
Annoying guy: In my particular line of work, I find that I must put on many different hats. I make copies, staple papers, and eat potato chips.