From Sean D. Kelly, chairman of Harvard's philosophy department, on Oct. 13, 2011—
"What I’m looking for at the moment is a good reference from Plato to make it clear how he understands the term. I remember that in the Thaeatetus there is discussion of knowledge as true belief with logos, and a natural account here might count logos as something like rational justification or explanation. And perhaps Glaukon’s request in the Republic for an explanation or account (logos) of the claim that Justice is a good in itself is a clue. But there must be other places where the term appears in Plato. Does anyone have them?"
See instances of logos under "Pl." (Plato) and "Id." (Idem ) in Liddell and Scott's A Greek-English Lexicon —
http://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.04.0057:entry=lo/gos .
(See also Liddell and Scott's "General List of Abbreviations"—
http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Asection%3D5 .)