Plethora refers to a very large amount or number of something. Plethora is most often used in the phrase "a plethora of."

// The hotel offers a plethora of amenities, including indoor and outdoor pools, a fitness center, and world-class dining.

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"In 1895, Japan won Taiwan from China in the Treaty of Shimonoseki, ending the Sino-Japanese War, and from 1895 to 1945, the island was a colony of Japan. The cultural influences of Japan persist to this day: in the tiny coffee shops with their enticing nutty aromas, in the plethora of Japanese-influenced sushi restaurants, and in Taiwan's highly efficient postal service and train systems." — Kim Liao, Where Every Ghost Has a Name: A Memoir of Taiwanese Independence, 2024

Plethora was first used in English to refer to a medical condition marked by an excess volume of blood or other bodily fluid, with associated swelling and redness. (Its Greek ancestor, the noun plēthṓra, refers to a similar excess, or to general fullness or crowdedness.) These days, plethora is most often used in general contexts to refer to a large number or amount of something. While plethora conveys a plural meaning, it is a grammatically singular word, which makes choosing whether to pair it with a singular or plural verb confusing. Both can be correctly used, however; choose the singular if you wish to emphasize the grouping or collection ("a plethora of books is available"), and choose the plural if you wish to emphasize the individual elements or components ("a plethora of books have been written on the subject").