I want to perform a lookup in a field, then restrict the editability of a field *only if* another field is not empty.

If the user selects an ItemCode from a drop-down, the Term field should lookup that term from the related file

What you need to do is add a validation by calculation is the field options.

Make the calculation something like this

if (IsEmpty(ItemCode),1, 0 )

I and that should (you might have to reverse the 1 and 0) make it so you can't alter the field onces there is an ItemCode.

Hate to say this, but don't listen to the above; when validating by calculation, you're only determining if the contents of a field match a calculated result.

One pretty good idea (if I do say so myself -- and I do):

Go to Field Format and uncheck "Allow entry into field", and take that field out of the Tab order. Now create a button that checks to make sure the field is editable (i.e. if the other field is not empty) and if so, goes to the field. That way, the button will be the only method in which anyone can enter data into the field.

I knew that.. i was ah... just testing you.