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)

The presence of two fetuses of different ages, not twins, in the uterus; due to the impregnation of two ova liberated at successive periods of ovulation.

Impregnation of woman already pregnant.

A second conception after a prior one, and before the birth of the first, by which two foetuses are growing at once in the same matrix.

A second conception while still with young.

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.