Meaning of internal reconstruction

inter'nal reconstruc'tion

Pronunciation: [key]
— Historical Ling. Historical Ling.
  1. the hypothetical reconstruction of an earlier stage of a language or of some part of it, as its phonology, by deductions from irregularities in its present structure, as the reconstruction of a stage in English when certain instances of r were related to s in a systematic way by comparing the pair was:were to other pairs, as lose:forlorn.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.