Firstly it's an Americanism.
Secondly it's a process not a symptom attributable to the parent with care.
Thirdly it is not generally recognised in the UK Courts and is very difficult to prove. However, Parental Alienation is recognised as a form of emotional abuse.
Fourthly the absent parent can add to the process by his/her own behaviour. Always late for contact? Don't make regular child support payments? Get angry at your ex in front of the children?
Fifthly it can be a conscious or unconscious form of brain washing of a vulnerable child. At worst it can totally alienate and destroy the child's relationship with the other parent. At least, it can make the child feel uncomfortable, unhappy, sad and isolated. For example:
"We can't afford to go on holiday because your dad spends all of his money on his new girlfriend."
This may well be true but does your child really need to know that? Do not burden your offspring with your resentments and dissatisfactions against their parent. Doing so is guaranteed to hurt your child!!!!!!!!!