In
linguistics, a
case form supposedly indicating that one thing
is another or is in a non-permanent
state. I have only ever seen it mentioned in the context of
Finnish, and have an idea the name was actually coined in Finnish, as
essiivi.
The basic ending is -na or -nä (depending on the preceding vowels: vowel harmony), but with some possible assimilation of stems ending in t or s which I don't think any of us need to know about. So poika 'boy', poikana 'as a boy'.
This is the theory, but in practice it seems to be another one of the abundance of Finnish local cases: kotona 'at home', takana 'at the back'; and is also used in some expressions of time: sunnuntaina 'on Sunday', jouluna 'at Christmas (joulu = Yule)'.