Signal magnitude area
I've found a proper definition thanks to the following article:
http://www.rehab.research.va.gov/jour/2013/509/jrrd-2012-12-0233.html

This is what I've edited Wikipedia with:
$$f_{sma}= {1 \over {T}} {\int_{0}^{T} |x(t)-a_x|+|y(t)-a_y|+|z(t)-a_z| dt}$$

(It wasn't because of text-search but images-search. Algorithms are sometimes surprising)