This article is from the alt.usage.english FAQ, by Mark Israel misrael@scripps.edu with numerous contributions by others.
Some uses of classical plurals as singulars in English are
undisputed: "opera", "stamina", "aspidistra". ("Opera", still used
as the plural of "opus", became singular in Vulgar Latin, and then
in Italian acquired the sense "musical drama", giving rise to the
English word.) "Agenda" once excited controversy but is now
accepted. Others are the subject of current controversy: "data"
(used by Winston Churchill!), "erotica", "insignia", "media",
"regalia", "trivia". Yet others are still widely stigmatized:
"bacteria", "candelabra", "criteria", "curricula", "memorabilia",
"phenomena", "strata".
 
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