How can a field definiton be imported, either via the admin or in code?
There is nothing in the interface but what you can do is make a temporary content type which uses the field(s) you wish to import, then export/import that content type to the target site.

Your new field(s) will then be available at the target site, make sure you attach them to other content types before you delete the temporary one or the field(s) will disappear.