X inactivation: The phenomenon in a female by which one X chromosome (either the maternally or paternally derived X) is randomly (by chance) inactivated in an early embryonic cell, with fixed inactivation of that same X in all cells descended from that cell. The change that occurs with X inactivation is epigenetic: it is a heritable change in gene function without a change in the sequence of the DNA.
X inactivation is not restricted to females. It also occurs in males with Klinefelter syndrome who have more than one X chromosome.
The phenomenon of X inactivation is also called lyonization after the English geneticist Mary Lyon (1925-) who first described it.