a dominant trait or disorder caused by a mutation in a gene on the X chromosome.
The phenotype is expressed in heterozygous females as well as in hemizygous
males (having only one X chromosome); affected males tend to have a more
severe phenotype than affected females.
Definition from: GeneReviews
from the University of Washington and the National Center for Biotechnology Information
Related discussion in the Handbook
See also Understanding Medical Terminology.