'Designer baby' is a term used by journalists to describe the fact that in the future we may be able to use genetic technologies to modify embryos and choose desirable or cosmetic characteristics. It is not a term used by scientists.
The definition given by the dictionary is: "A baby whose genetic makeup has been artificially selected by genetic engineering combined with in vitro fertilization to ensure the presence or absence of particular genes or characteristics."
So, “Designer babies” are the children that have been genetically engineered in the womb to have certain desired qualities. During a pre-implantation genetic diagnosis or embryo screening, a scientist would be able to tell what physical characteristics a child will grow to have; as well as whether or not this child is at risk of developing or will develop genetic disorders such as autism, down syndrome, Huntington’s disease, Alzheimer’s and many other disorders such as these.