pro·estrus noun orpro·estrumalsopro·oestrusorpro·oestrum\“+\ (pluralproestrusesorproestrums) Etymology: New Latin, from pro- (I) + estrus, estrum, oestrus, oestrum : a preparatory period immediately preceding estrus and characterized by growth of graafian follicles, increased estrogenic activity, and alteration of uterine and vaginal mucosa