c.1600, from L., lit. "from the beginning", from ablative case of initium "entrance, beginning", related to verb inire "to go into, enter upon, begin".
Abbr.: Ab init. Ab initio is used in several contexts:
- when describing literature: told from the beginning as opposed to in medias res (meaning starting in the middle of the story)
- when describing a subject or a module, say when a person is learning French from the beginning, he is said to be a student of French ab initio
- as a legal term: 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. A judicial declaration of the invalidity of a marriage ab initio is a nullity.
- in science: 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 electrodynamics. 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.
- in chemistry: an abbreviation referring to ab initio quantum chemistry methods.
- in biophysics: a method for the prediction of protein structures in protein folding
- in aviation: The very first stage of flight training.
- as part of some educational qualifications: foreign languages may be taken ab initio (for beginners, of a language) during the two year IB period. This as compared to level B which assumes some level of proficiency.
- in computing: ab initio is an extract, transform, load tool used to manipulate data.
- in bioinformatics: ab initio is a term used to define methods for making predictions about biological features using only a computational model without extrinsic comparison to existing data. In this context, it may be sometimes interchangeable with the Latin term de novo.
In law, void means of no legal effect. An action, document or transaction which is void is of no legal effect whatsoever: an absolute nullity - the law treats it as if it had never existed or happened.
The term "void ab initio", which means "to be treated as invalid from the outset", comes from adding the Latin phrase "ab initio" as a qualifier. For example, in many jurisdictions where a person signs a contract under duress, that contract is treated as being "void ab initio".
A proposition in law that a court's jurisdiction, a certain document which purports to affect legal rights, or an act which purports to affect legal rights, is or was null and void from the start, from its beginning, because of some vitiating element.
Typically, documents or acts which are void ab initio cannot be fixed and where 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 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 this effect and not, as with void ab initio, as if they never existed.
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; that 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.
See also 
|Look up ab initio in Wiktionary, the free dictionary.|