ab initio是拉丁文，意思是“自开始之时”或“从头开始”，可直接译为“自始”。在法律英语中，ab initio的使用频率很高，是比较常见的一个词。这个用语最常见与void一词一起出现，即你问题中的void ab initio，形容一开始便无效的交易或行为（如合约或法律程序）。类似的搭配还有ab initio mundi，有史以来。

《Lexisnexis英汉法律词典》中对void ab initio的解释如下：

Void ab initio  

由最初开始即属无效

Lat – of no effect from the beginning. A contract, a marriage, a disposition or other transaction void ab initio is treated as never having had legal effect; this is the usual meaning of ‘void’. Court proceedings are unnecessary to deprive a contract void ab initio of effect; rights and liabilities will not be recognised to the extent that the existence of a void contract must be asserted to prove them. An illegal agreement or transaction is void ab initio: Wa Lee Finance Co Ltd v Staryork Investment Ltd & Ors (HCMP 2242/2000, unreported). If the condition precedent is not fulfilled the contract will be void ab initio: Hang Seng Goldsmith Co (A Firm) v American International Assurance Co Ltd [1985] 1 HKC 146. A contract which is void ab initio can have no contractual effect, though it may be followed by a transaction which is itself legally operative, such as a conveyance of property. In relation to sale of goods, where the contract is void ab initio, the buyer may recover any part of the price which he has paid. Rescission of a contract may mean making it null and void ab initio: Johnson v Agnew [1980] AC 367, [1979] 1 All ER 883 (HL). See also Void; Voidable; Void contract. 

拉丁语 – 从开始起无效力。由最初开始即属无效的合约、婚姻、处置或其他交易被视为从来没有法律效力，这是「无效」一用语的普遍涵义。不需任何法律程序已可使由最初开始即属无效的合约失去法律效力；有关的权利及法律责任将不获承认，致使该无效合约的存在必须予以肯定，以证明该等权利及法律责任。所有非法协议或交易均属由最初开始即属无效：Wa Lee Finance Co Ltd v Staryork Investment Ltd & Ors（高院杂项案件2000年第2242号，未经汇报）。如未能履行先决条件，则有关的合约会由最初开始即属无效：Hang Seng Goldsmith Co (A Firm) v American International Assurance Co Ltd [1985] 1 HKC 146。由最初开始即属无效的合约不可有法律效力，纵使于该合约之后，出现例如财产转让之类在法律上本身可施行的交易。就售卖货品而言．如合约是由最初开始即属无效，则买方可追讨已支付金额的任何部分。撤销合约可指令该合约由最初开始即属无效：Johnson v Agnew [1980] AC 367, [1979] 1 All ER 883（上议院）。