c. 1600, from Latin, lit. 'from the beginning', from ablative case of initium "entrance, beginning", related to verb inire "to go into, enter upon, begin".
Ab initio (abbreviation: ab init.) is used in several contexts, including the following:
In law, ab initio refers to something being the case from the start or from the instant of the act rather than from when the court declared it so. For instance, the term "void ab initio" means "to be treated as invalid from the outset." E.g., in many jurisdictions, if a person signs a contract under duress, that contract is treated as being "void ab initio".
Typically, documents or acts which are void ab initio cannot be fixed and if a jurisdiction, a document or an act is so declared at law to be void ab initio, the parties are returned to their respective positions that they were at the beginning of the event.
"Void ab initio" is often contrasted with "voidable", such documents which become void only as of the date of the judicial declaration to that effect.
An insurer facing a claim from an insured who had deceived the insurer on a material fact would claim that the insurance contract was void ab initio; it was null and void from the beginning and that since there was no legally enforceable contract, the insurer ought not to have to pay.
In general, a calculation is said to be ab initio (or "from first principles") if it relies on basic and established laws of nature without additional assumptions or special models.
For example, an ab initio calculation of the properties of liquid water might start with the properties of the constituent hydrogen and oxygen atoms and the laws of electrostatics and quantum mechanics. From these basics, the properties of isolated individual water molecules would be derived, followed by computations of the interactions of larger and larger groups of water molecules, until the bulk properties of water had been determined.