When a journal applies to be included in MEDLINE (PubMed’s primary database), it is assigned an "NLM Title Abbreviation." This is the direct descendant of the old Index Medicus abbreviation.

To save space on physical library cards and printed pages, librarians developed a strict system of abbreviations for every journal they indexed. Instead of writing out The New England Journal of Medicine , they condensed it to N Engl J Med . Today, Index Medicus is gone, but its abbreviation system lives on through its digital successor: PubMed and the NLM Catalog (National Library of Medicine).

You might see N Engl J Med and know exactly what it stands for. But what about Zentralbl Gynakol ? Or Khirurgiia (Mosk) ?