Three people I have recently spoken to (an ex XC/TF coach, an ex XC/TF athlete, and an experienced runner who ran with Kenyans and talked to a lot of elites) all have referred to a tempo run as a fartlek. When I asked each of them independently what a tempo run is they would say "Oh a fartlek, speed play, it was invented by the Russians." Something along those lines.
Have I been misinformed throughout my running career or are they two different things like I had thought?