Although dictionaries do not provide a definition for "sex change", it means any change and transformation in a male, female or genderless in such a way that the current state is different from the previous one.
Medically speaking, sex change is different from "genetic modification" or a "change of chromosomes". In other words, sex change in medical terminology involves "an operation on the reproductive organs" which applies to sex change in both the androgynous or hermaphrodites and transsexuals.
Shiite jurisprudents have provided various definitions for sex change but it can be defined as "change or transformation in one's gender from male to female, from female to male, from genderless to male, from genderless to female, from male to genderless or from female to genderless."
We shall answer the above question in three sections:
1-Literal meaning of sex change
Although sex (jins) change has not been defined in dictionaries but literally it refers to a change and transformation in such a way that the current state is different from the previous state. Sex (jins) in its literal meaning in Arabic and Farsi refers to an identity which includes various kinds of humans and animals. It also means “goods” or “stock”. “Jins” in the sense of “male and female” is a meaning that has come into being in a secondary phase; otherwise it is not the primary and original meaning of “jins”.
Sexuality (jinsiyah) is the mode or the state of the jins.
Therefore sex change refers to any change in such a way that the current state is different from the previous one.
2-Sex change in medical terms
It seems that "sex change" in medical terms is not much different from its literal meaning and what is generally perceived by people. It means transformation or reassignment of reproductive organs in such a way that a female transforms into a male or vice versa. Transformation of a hermaphrodite or a genderless into male or female is also an example of "sex change". In essence, "sex change" applies to any transformation, change or reassignment in one's gender where his current state is different from his previous state in terms of being a man or a woman.
Therefore, sex change is different from "genetic modification" or "change in chromosomes” because it involves a change in one’s gender not a change in his "genes" or "chromosomes". It is obvious however that along with changes or modifications in the genes or chromosomes the person undergoes behavioral changes as well but the new state which has been brought about as a result of the modification is never called “sex change”.
In other words, sex change in medical terminology involves "surgery on reproductive organs" which applies to sex change in androgynous and hermaphrodites and in transsexuals in such a manner that sex change occurs only by means of surgery. In practice, sex change is closely associated with "the history of modern surgery". When surgeries on heart, kidney, eye, stomach and similar organs became popular, reproductive organs also became candidates for such surgeries. But surgeries on reproductive organs have always been tackled with extra sensitivity and attention since this operation leads to change in one's gender. In short, sex change in medical terms is different from genetic modification and is a surgery on reproductive organs in a way such that a female turns to a male or a male to a female or a hermaphrodite turns to either a male or a female.
3-Sex change in jurisprudential terms
Sex change does not refer to a particular Shari’ah concept i.e. jurisprudents have not provided a specific jurisprudential definition for sex change to be distinct from literal and common meaning. In fact by sex change they refer to its literal and regular meaning. Referring to this meaning, Ayatollah Meshkini says: "Sex change is clear and tangible in its literal meaning; the object of putting forth this topic is to explain the Islamic law regarding a man who has turned into a woman or a woman who has turned into a man." 
Only a few Shiite jurisprudents have discussed the meaning of sex change such as the following:
Ayatollah Muhammad Asef Mohseni says in this regard:
"Sex change is turning a man into a woman or vice versa. In the first case i.e. changing a male into female, the man's reproductive organs are removed and replaced by a vagina and breasts are augmented; in the second case i.e. changing a female to male, the breasts and female reproductive organs are surgically removed and replaced by a male reproductive organ. In both cases surgery may be accompanied by psychotherapy and hormone therapy."
Some Shiite jurisprudents use the term "gender reassignment" instead of "sex change". Ayatollah Muhammad Sadr says in this regard:
"Gender reassignment may occur through modern surgery where a man turns into a woman or a woman turns into a man, something which we termed earlier as gender reassignment".
Else where the same jurisprudent defines "gender reassignment" as under: "By gender reassignment we mean that a man changes his gender from male to female or vice versa."
Given these statements, the jurisprudents' definition of "sex change" or "sex reassignment" refers only to change of "male to female" and "female to male". In other words, there is no mention made of an androgynous (neutral) changing into male or female.
Although Imam Khomeini does not provide a clear definition for "sex change" his statement also includes reassignment of an “androgynous to male/female". Imam Khomeini says: "It appears from the religious texts that it is not haram to change a man’s gender to female and a woman’s gender to male or an androgynous’ gender to male or female.”
In addition, this probability should not be ignored that some healthy people may turn into genderless as a result of a medical surgery. However, this type of sex change i.e. change of male/female to genderless rarely occurs since there is no rational motive or wisdom behind it but it is probable that due to a medical error a healthy person may turn into a genderless human being.
Having provided this explanation, sex change may be defined as: "Change or reassignment of one's gender from male to female, female to male, genderless to male, genderless to female, male to genderless or female to genderless."
 Meshkini,Ali, Mustalahat al-Feqh,p.153,Qom, Al-Hadi, 1999
 Mohseni, Muhmmad Asef, al-Feqh val Masa'el al-Tebbeyya
 Sadr, Muhammad Sadeq, Mawara' al-Feqh (Bairut, Darul Adhwa, 1966) vol.6, p.133
 Sadr, Muhammad Sadeq, Manhaj al-Salehin, vol.3, p.642, Bairut, Darul Adhwa,2003
 Imam Khomeini, Tahrir al-Wasila,vol.2, p.588, Qom, Esma'ilian, 1987
 See Sadr, Muhammad Sadeq, Manhaj al-Salehin, vol.3, p.642