Ex tunc is a legal term derived from Latin, and means "from the outset". It can be contrasted with ex nunc

Ex nunc

Ex nunc is a Latin phrase meaning from now on. Used as a legal term to signify that something is valid only for the future and not the past. The opposite is ex tunc....

, which means "from now on". One example usage of the term can be found in contract law, where avoidance of a contract can lead to it either being void ex nunc, i.e. from then on, or ex tunc, in which case it is treated as though it had never come into existence.