Zona hatching is a phenomenon occurring during prenatal development. Prior to this event, the predecessor of the embryo, in the form of a blastocyst, is surrounded by a glycoprotein sphere called the zona pellucida. To be able to perform implantation on the uterine wall, the blastocyst first needs to get rid of the zona pellucida. This lysis of the zona pellucida is called the zona hatching.This happens at the end of day four.
Uses of the term
In some situations, the term "hatching" is used only for artificial ways to free the blastocyst from the zona pellucida, and not the natural lysis of it. An example of such artificial way is assisted zona hatching.
References
Zona hatching Wikipedia(Text) CC BY-SA