Dictionary.net

The fertilization and subsequent development of an ovum when a fetus is already present in the uterus, a result of fertilization of ova during different ovulatory cycles and yielding fetuses of different ages. (Dorland, 28th ed)

Impregnation of woman already pregnant.

Fertilization of two ova formed at different menstrual periods.

The supposed fecundation (now held to be impossible) of an ovule proceeding from one ovulation while a fecundated ovum proceeding from a previous ovulation is still in course of uterogestation. See superfecundation.