Trifle, "form field" is 100% correct for those who design WebPages or set up sites. I just asked my wife, and she immediately knew the term. However, I did not, so this definition might not be the best of the average user. But my question is this: are we looking for a translation for a programmer, a user, or both?
For programmers, I think your definition is best.
On paper forms, we are usually told things such as these:
Fill in all the blanks
Check all the boxes
Now, notice the form for LEO, that I will use to "post" this:
'Translation' or 'Comment', as well as 'Name' are obligatory, the rest of the data are optional.
As you can see, the user does not see the word "field" or "form". The site assumes that we understand that the "boxes", big or small, must be filled in if they are "obligatory".
I'm not sure (and I'm interested to hear comments from other), but I think for the user the words "blanks", "spaces", "boxes" work better.