By the end of the first decade of the 21st century, spondyloarthritis studies have accumulated a certain number of terms that are obsolete, but used by physicians in their everyday speech, on the one hand, and a great variety of different definitions, on the other hand. In January 2014, the first organizational meeting of the Expert Group on Spondyloarthritis, Association of Rheumatologists of Russia, decided that its primary task should be to order the terminology used in this area. The authors primarily collected the terms, which had been already used in medical vocabulary, and then divided them into two categories: obsolete definitions and terms to be finalized and unified. This publication gives guidelines for using the medical terms relevant to spondyloarthritis and separately discusses how to correctly write the term sacroiliitis.