פירוש על ברכות 29:12

In Rabbinic law a distinction is often drawn between the validity of an act which one contemplates doing and its validity after it has been performed; and it is sometimes decided that though the former (לְכַתְּחִלָּה "as at the beginning," ab initio) is illegitimate, nevertheless the accomplished fact דִיעֲבַד = דְּאִיעֲבַד "that which has been done," post factum) may be considered as complying with the requirements of the law. See Mielziner, p. 197 ; Bacher, Terminologie, ii. p. 232.