Referendums have been in the news lately (Crimea, Scotland), or should that be referenda? The OED says that both plural forms are used in more or less equal measure these days. However, it notes that the form referenda is often deprecated in usage guides (Fowler prefers referendums, for instance).
The word referendum comes from Latin, but it was not a noun in Latin, as it is in English. It is the gerund, or neuter gerundive form of the verb referre, meaning to refer. Referenda is a Latin plural gerundive and would mean, pedantically speaking, 'things to be referred'. There is no gerundive in English, although words ending in -able (lovable, referrable etc) have a similar sense. English uses gerunds a lot, however. A gerund is a verbal noun, and in English gerunds end in -ing. In the sentences 'Smoking is bad for you' and 'I hope you don't mind my smoking', smoking is a gerund in both sentences (ie a noun, which is why you can say 'my smoking', although 'me smoking' is just as likely to be heard today).