Writing or saying something once
when it should have occurred twice
. The Greek
root is haplo-
Some words have been shortened at some point in their history to avoid the repetition of a syllable. So the Greek compounds that ought to have become symbol-ology and idol-olatry are in fact now symbology and idolatry.
Haplology also occurs in manuscripts, where it is simply a mistake in reading. A scribe copying a line gets to a word which happens to occur again just below it. In the new copy the upper line switches to the lower line at that point, omitting one instance of the repeated word, and a line's worth of text.