Plain English Meaning
The lowest altitude you may descend to on an RNAV (GPS) approach when the GPS is only steering you side-to-side, not guiding you up and down. You level off at this altitude and look for the runway.
Full Definition
The lateral-navigation-only minimum descent altitude published on an RNAV (GPS) approach chart. LNAV minimums apply when the aircraft is using GPS for course guidance only, with no vertical guidance from the navigation system. The pilot descends in a non-precision step-down manner to the published MDA and may not go below it unless the required visual references and other landing criteria are met.