What is the meaning of the French phrase "avant la lettre" which I
have seen used in scholarly writing?  It is not "before the letter,"
the literal translation, but is some kind of idiom.  Thanks.

Hello viseu and thank you for your question.

The French phrase "avant la lettre" means "before the term (or phrase) existed"

For example:

"She is driven by these sharp emotional changes because as an artist
she is dealing with the subject before it was defined, the medium
before it existed. In this sense, she is an artist of things avant la
lettre."
http://www.paris.fr/musees/MAMVP/expositions/schneider/schneider_cat_scherf_ang.htm

and a translation of a book title:

"Votiefgeschenken: epidemiologie avant la lettre"
"Votive offerings: epidemiology before the term existed"
http://www.volkskunde.be/Inhoud%202000,%203.htm

also see:

"avant la lettre [F] foreign term : before the letter: before a
(specified) name existed"
http://www.orbilat.com/Languages/French/Vocabulary/French-International.html

Thank you for your question, and if you need any clarification of my
answer, do not hesitate to ask before rating my answer.

Very best regards

THX1138

Search strategy included:
"avant la lettre"  before existed
://www.google.com/search?q=+%22avant+la+lettre%22++before+existed&hl=en&lr=lang_en&ie=UTF-8&safe=off&as_qdr=all&start=10&sa=N

Brilliant, thank you!  Thanks also for showing me where to find such
phrases in the future.

Hello again viseu.

Just a note to say thank you for the five star rating and generous tip.

Very best regards

THX1138