It is normally the Outer Marker or equivalent point (such as DME distance or a crossing radial/bearing). If no such point exists, using 1000' above aerodrome elevation is not uncommon.
An instrument approach may not be continued past this point unless the weather are at or above minima (rvr/vis for precision approaches, and also cieling for non-precision approaches). Once this point is passed, the approach may be continued to minima regardless of reported weather conditions.
Edit: mixed the cards there - cieling is required for planning purposes but is not part of the "commencement or continuation of approach". Mea culpa.
Last edited by bfisk; 6th November 2011 at 16:11.