When was the first official use of the term 'minifigure' or 'minifig'?
I kept searching and found out for myself :). The US magazine from 1979 calls them Mini-Figures, even the version without arms.

http://www.peeron.com/catalogs/1979/medium/6/?id=97
http://www.peeron.com/catalogs/1979/medium/10/?id=97