A textual critic is often faced with
a dilemma: the variants in all the
surviving witnesses are for some
reason unsatisfactory, and he or she is forced to make an
educated guess about the proper reading of the text. When a
textual critic proposes a reading that doesn't appear in any of
the witnesses, it's called a conjectural emendation.
From the Guide to Literary Terms by Jack Lynch.
Please send comments to Jack Lynch.
Note: This guide is still in the early stages of development.
Three question marks mean I have to write more on the subject. Bear with me.