i think jeff is right, over time, the 'lap' was dropped.
i have 3 software editing suites, and 'lap dissolve' appears nowhere, just 'dissolve.'
i only see lap dissolve in older screenplays.
just looked in THE COMPLETE GUIDE TO STANDARD SCRIPT FORMATS by cole/haag. lap dissolve appears nowhere.
looked in the glossary of another book, had a definition for lap dance, but not for lap dissolve.