Referenda & Referendums
Definition: a public vote on a particular issue
Latin has given English a sizable portion of its vocabulary, and one of the reasons that so many of our words are descended from that language is that they have entered our tongue at a number of distinct points. Some, such as butter, date back to the Roman invasion of Britain. Others came to us in the middle ages, from the Norman Conquest. Others still did not arrive until the 19th century, often as part of an expanding scientific vocabulary. Referendum is one of those late arrivals, initially used to describe a vote on the Swiss constitution at that time. Both referendums and referenda are correct.
SYLLABUSES & SYLLABI
Syllabus comes to English from the Latin sillybus, which actually has nothing to do with buses or silliness (sillybus refers to a label for a book). Both forms of the plural are acceptable, although people might look askance at you if you use syllabi.
Gymnasiums & Gymnasia
Very few people use gymnasia as the plural of gymnasium anymore, but it can come in handy if you want to confuse people. And if you desire still more obscure information about this word, know that it can be traced to a Greek word meaning “to exercise naked” (gymnazein). The naked aspect of this word’s history has been largely set aside in English, although Nathan Bailey, in his 1727 Universal Etymological English Dictionary, defined the word gymnologize as “to dispute naked, or like an Indian Philosopher.”
Miasmas & Miasmata & Miasms
Most words have a single plural form, while others feel the need to have two. Others are more gluttonous still (such as octopus, the plural of which may be octopi, octopuses, or octopodes), and require three different ways of pluralizing. Miasma is one of those triplets. So if you have more than one of these vaporous exhalations (and we hope you never do), they may be described as miasmas, miasmata, or miasms.
Terminuses & Termini
Given that the above two words are obscure variant plurals of a somewhat obscure singular form (terminus), it is quite unlikely that you will have need of distinguishing between them. But seeing as how the joy of unexpected knowledge is not regulated by need, we have included them anyway. You may use either terminuses or termini (but not terminusses) with ferocious and joyful impunity.
Memorandums & Memoranda
Many usage guides have taken pains to warn their readers of a potential problem with the plural of memorandum. This is not whether or not to use memorandums or memoranda (either is fine), but rather to avoid using memorandas. Some words with similar Latinate endings in English will allow an a ending to take an s (such as agendas), but memorandas is not among
Virtuosos & Virtuosi
Both virtuosos and virtuosi may be found as the plural form of virtuouso, although the former is more common than the latter. Virtuosi is also viewed by a number of usage guides as being overly pedantic, and many will recommend virtuosos instead. So if your desideratum (which is only pluralized as desiderata) is to use the sort of words that usage guides think of as overly pedantic, well, then virtuosi is the word for you.
Appendices & Appendixes
Some people are of the opinion that when giving the plural of appendix one form is appropriate for certain contexts and not for others (stating, for instance, that appendices should be when referring to texts, and appendixes for non-textual things). We have a considerable body of written evidence indicating that these plurals are used interchangeably, so decide which one you are more comfortable with, and use it at will.