A very large amount of something: an amount that is much greater than what is necessary.
1) A plethora of books has been written upon the subject.
2) My biology textbook is helpfully illustrated with a plethora of illustrations.
Medieval Latin, from Greek plēthora which literally means 'fullness'.
abundance, cornocupia, feast, plentitude, plenty, wealth
deficiency, inadequacy, insufficiency, undersupply.