Decision Height was generally used for the definition of the criteria for aborting, i.e. initiating missed approach, during an approach with vertical guidance, i.e. when on a glide slope. The FAA now wants to call it Decision Altitude. The idea is that you follow the glideslope, and initiate missed approach at the DH if the runway environment is not at that moment visible.
Minimum Descent Altitude (MDA) was/is used when an approach did not include a glide slope, i.e. the basis would be to descend from the fix to the MDA. Then motor towards the airport at that height until the decision point, which in this case is normally defined (on a straight in) as not seeing the runway in time to perform a reasonable landing.
A reference from the FAA Aeronautical Information Manual:
From
http://www.faa.gov/ATpubs/AIM/Chap5/aim0504.html#5-4-19 :
"4. Chart Terminology
(a) Decision Altitude (DA) replaces the familiar term Decision Height (DH). DA conforms to the international convention where altitudes relate to MSL and heights relate to AGL. DA will eventually be published for other types of instrument approach procedures with vertical guidance, as well. DA indicates to the pilot that the published descent profile is flown to the DA (MSL), where a missed approach will be initiated if visual references for landing are not established. Obstacle clearance is provided to allow a momentary descent below DA while transitioning from the final approach to the missed approach. The aircraft is expected to follow the missed instructions while continuing along the published final approach course to at least the published runway threshold waypoint or MAP (if not at the threshold) before executing any turns.
(b) Minimum Descent Altitude (MDA) has been in use for many years, and will continue to be used for the LNAV only and circling procedures. "
A Cat I ILS then, even if not using a radalt, would have a Decision Height. These days, of course, almost no one operating to Cat I or below does not have a radalt, so it commonly is interpreted as a radalt height.
The EFIS, of course, can display the limit either on the basis of radalt or baro alt, so may incorrectly use the term MDA for the selection of baro for the bug height. This would thus also work for a circling approach, as the EFIS will command level flight at the MDA in that case. If you are on a glideslope, you should be initiating missed approach when reaching bug height anyway.
Cheers.