Immersion is a discontinued feature of Duolingo. It allowed users to practice the language they were learning by helping to translate real-world documents from that language into one they already knew.[1] This feature was only available for few courses (see here).
The Duolingo CEO announced in June 2015 that they have stopped developing immersion[2]. The last course to gain an Immersion tab was the English for Romanian speakers course (which reached beta phase on 2014/02/04). Around December 2015, Duolingo started an A/B test to evaluate Immersion.
After much speculation, Immersion was withdrawn on 19th January 2017[3].
References
de:Immersion