According to http://www.sheldonbrown.com/gloss_m.html
and what I understand:
"A style of lady's frame in which the "top tube" consists of a pair of small diameter tubes running more-or-less straight from the upper head lug, past the seat tube, and on to the rear fork ends. A mixte frame thus has 3 sets of rear stays, instead of the usual two. A variant on the mixte uses a single, full sized top tube running from the upper head tube to the seat tube, but retains the middle set of stays. A lady's type bike that lacks the middle pair of stays is not a mixte."
As seen on this '83 Raleigh Clubman: