what is it called when a word's plural form ends in -i?

Question

what linquistic term is used to describe a word who's plural form ends in -i as in thesauri or radii?

Answer 1

Unfortunately, the only terms we have are things like "irregular plurals ending in i".

A note or two on these:
It is true that some of them are or may be based on mistakes. But that does not automatically make them incorrect ENGLISH plurals. For that matter, the fact that a noun originally derives from the Greek, rather than Latin, does not decide the matter. This is, in part, because many of our Greek-based words were borrowed into English THROUGH Latin. "Octopus" is one example. In fact, I too would recommend the simple Anglicized plural "octopuses" [as, I see, Yahoo!Answers spell check would!], but cannot say "octopi" is unacceptable. When in doubt, check the dictionary (which in this case allows BOTH of these plural forms) -- http://dictionary.reference.com/browse/octopus

Here, by the way, is a list of Latin -us singulars that do NOT use -i for the plural --
http://www.alt-usage-english.org/excerpts/fxplural.html

Answer 2

there is no known term .. most of the words pluralized with the letter 'i' are incorrect or meant to be humorous .. such as octopus = octopi in fact is incorrect octopus comes from the Greek .. however it would take forever to explain so have a look at this site it will answer your question, I hope

http://www.askoxford.com/asktheexperts/faq/aboutgrammar/plurals?view=uk

Answer 3

The PIE contraction of vowels at a morpheme boundary merged with pre-existing long vowels.
Proto-Indo European (linguistics)

Answer 4

it is called a hypercorrection. there are 4 different reasons it is necessary.

Most likely it is because the word has a usage which is correct in another language but is not required in English.