Ukemi translates to receiving body. Uke is basically the guy who gets done and the techniques he uses to do that safely are called ukemi.
So purists would argue that without a doer (tori/nage/shite) there can be no doee (uke) and therefore no ukemi.
However, we do practice our ukemi skills solo and also use those same skills when the tori/uke distinction gets lost in sparring. I'm perfectly happy calling safe falling under any condition ukemi.