There are two senses or contexts to this word.
The art context is the easier one: Aktionismus = actionism
See for example:http://www.tate.org.uk/collections/glossary/d...http://www.artnet.com/magazine_pre2000/news/w...http://en.wikipedia.org/wiki/Viennese_Actionism
The psychological context---i.e. in the sense of "blinder Aktionismus"---is tougher, as reflected by the many forum entries about it so far. Here's one native speaker's opinions on those entries:
"Actionism" is definitely wrong here.
"Knee-jerk" action/activity/whatever is close but perhaps a little too short-term.
"Aimless activism" is frighteningly off the mark.
"Doing things for the sake of doing things" is apparently in Langenscheidt but lacks urgency and is a little clunky---especially when you're trying to translate a title like "Zwischen Aktionismus und lähmender Blockade," as I was this morning.
"Need to be seen to be doing something" is even clunkier, plus it's describing an activity that's inherently calculating---there's nothing "blind" about it.
So here, after much googling and pondering, is my suggested translation: "impulsive action" as described here:http://books.google.com/books?id=WcgCMvH8lFAC...