ideo

ideo is id plus the old ablative ending -eo, "on account of that". Unlike most connectives it points FORWARDS at least as often as backwards: it very commonly sets up a following ut or quia clause, ideo ... ut, "for this reason, namely in order that", and the reader is meant to wait for the second half.

Set it against its neighbours. ergo and igitur draw a conclusion from what has just been said. itaque marks a consequence in the flow of a narrative. ideo names a cause, and it names it as a cause. It is also a good deal commoner in scholastic and medieval Latin than in Cicero, where propterea and idcirco share the work, and that later usage is part of why it ranks as high as it does in our corpus.