fartlek
Concise Oxford English Dictionary © 2008 Oxford University Press:
fartlek /ˈfɑːtlɛk/
▶noun Athletics a system of training for distance runners in which the terrain and pace are continually varied.
– origin 1940s: from Swed., from fart ‘speed’ + lek ‘play’.
'fartlek' also found in these Oxford entries:

