The question may come from a student pilot or PPL,
1)DA decision altitude- the altitude on an ILS approach at which if you do not have the required visual reference to land, you go around.
2)DH decision height- the same as above except with the altimeter set to read height above threshold (QFE) or from a radio altimeter if using this(eg CAT2 or CAT3 approach)
3) MDA minimum descent altitude- the minimum altitude you fly down to on a non precision approach at which (without visual reference) you either go around or maintain till missed approach point or final descent point depending on SOPs.
Just to add confusion, some non precision approaches now publish DAs which can be used if flying a constant descent approach.Also, If you fly a bus, the MDA box is used to set DH if using a barometric minimum.