My significant other is training to be a librarian and was amused to come across the word 'documentalist' in one of her academic texts recently.
The OED Online defines a documentalist somewhat laconically as "a person engaged in documentation"; Wikipedia adds that "when the field of documentation changed its name to information science, the term information specialist often replaced the term documentalist". Shouldn't that be information scientist?
I'm not sure exactly when documentation became known as information science, but 'documentalist' can't have had long in the limelight as the OED dates it from as late as 1939 (and Merriam-Webster agrees).
As an aside: documentalists are not to be confused with mentalists such as Derren Brown...
1 day ago