A simple question that has sparked some debate, and I couldn't find a concrete answer anywhere. There seems to be two camps: The word plethora indicates plural, so therefore it should be "There are a plethora"; and the other camp says that there is only one plethora (which contains multiple), so it should be "There is a plethora".
I've seen many examples of both. Is there a consensus, or is it just one of those things that can go either way?
Best Answer
This look at the matter of the grammatical plurality of single groups of plural items is well worth reading in full.
To summarise:
I'll also note that John Cowan (who often remembers such things) is quoted thus:
My own opinion, is that the singular is easily understood as attaching to the noun that describes the group, just as the plural is easily understood as attaching to the individual members of the group. Neither clash horribly for most readers, though some do hold strong opinions.
My own use doesn't even side firmly with one or the other (my preferences here are given not as prescriptive decrees, but using myself as an example to demonstrate that usage may not even be firmly on one side or the other with a given individual).
If a phrase clearly refers to the individuals, I would strongly favour the plural:
Some clearly to the group:
Many could be interpreted either way:
In which I am likely to just mentally place in one of the first two categories, and write accordingly. Note that even with the first two, my strongly favouring one or the other does not mean I find other choices to be incorrect in others' writing.
From what I say above, you might expect that I would always favour the singular if the individuals aren't mentioned (even if I wouldn't "correct" another for doing otherwise):
But a group almost always at least suggests its members, and this question deals with one example of that, but there are others.
There are technical contexts where I would much more strongly suggest that expressions specific to the group or to the individuals be distinguished: The distinction between sets and their items in mathematics, or between collections and their contents in object oriented computer programming. Even here, I wouldn't so much say that a given form was wrong, just that there is a heavy benefit for precision and often you are working to help readers understand that very distinction, so an extra bit of prissiness may pay off.