"void ab initio" : A contract formed under duress.

Ab initio 

Means "from the beginning" in legal contexts, it refers to somthing that is considered to have been the case or legally valid from the outset as though no intervening period exists.

Void ab initio : refers to somthing that is considered in valid from the very beging.

For example: a contract enter into under duress or fraud may be valid ab initio meaning it was never legally binding.

Trespass ab initio: A legal doctrine where someone who lawfully enters on to anothers property later abuses that privilege, and their entry is retroactively considered trespass from the outset.

Ownership ab initio: use when ownership or right to property are deemed to have existed from the very start of a particular legal metter.

Contract ab initio: refer to an agreement being legally effective from the moment if it's formation though sometimes challenged if there's a legal defect present at the beginning.

Conclusion: 

these phrases emphasis that the issue in question is considered from the point of origin or inception, without regard for any subsequent devlopment.