Some of this is stating the proverbial obvious but, for the benefit of the uninitiated, please bear with me 😀
Definition taken from Wikipedia – see http://en.wikipedia.org/wiki/Third_culture_kid
Third Culture Kids or Trans-Culture Kids, (abbreviated TCKs or 3CKs,) sometimes also called Global Nomads, “refers to someone who, as a child, has spent a significant period of time in one or more culture(s) other than his or her own, thus integrating elements of those cultures and their own birth culture, into a third culture”.
Since the term was coined by sociologist Ruth Hill Useem in the 1960s, TCKs have become a heavily studied global subculture. TCKs tend to have more in common with one another, regardless of nationality, than they do with non-TCKs from their own country.
An adult who was a TCK. I.e, an Adult Third Culture Kid. This term appears to have been coined because there was some confusion about the term ‘Third Culture Kid’ only referring to children and not adults.
Definition taken from http://www.crossculturalkid.org/blog/cross-cultural-kids/. I believe this term has been coined by Ruth Van Reken and Paulette Bethel.
A Cross-Cultural Kid (CCK) is a person who has lived in—or meaningfully interacted with—two or more cultural environments for a significant period of time during developmental years.
Definition taken from http://en.wikipedia.org/wiki/Missionary_kid
Missionary Kids (or MKs) are the children of missionary parents, and thus most were born and/or raised abroad (that is, on the “mission-field”). They can be a subset of Third Culture Kids (TCKs), but not always.