Is it possible to index a calculation field that uses another field from another database?

e.g.

In the main database I have a field call "A" which has this calculation:

Max(Academic History::Term)

Any help would be greatly appreciated.

No, because Filemaker doesn't update it's index whenever related records and unstored calculations change. So, the index would be wrong after a change to the record in the related file. It would be a major undertaking and would probably slow Filemaker down way too much for it to track all these things.

The best you can do is create another field that is set to the value of your calculated field by using a Replace command or a Looping script. That field can be indexed, and you can use it in finds and sorts. You just have to update it regularly.