The correct definition of a third culture kid is the definition used in the book "Third Culture Kids, The Experience of Growing up Among Worlds" written by David Pollock and Ruth van Reken.
“A Third Culture Kid (TCK) is a person who has spent a significant part of his or her developmental years outside the parents’ culture. The TCK frequently builds relationships to all of the cultures, while not having full ownership in any. Although elements from each culture may be assimilated into the TCK’s life experience, the sense of belonging is in relationship to others of similar background.”
I really like this short trailer "Les Passagers: a TCK story". It is a documentary special about Third Culture Kids (TCKs) and their journey to find where they belong. Thanks Kalen and Aga Magdolen for making this useful documentary! An eye opener.