Definition
DA/MDA is the combined term for the lowest altitude to which a pilot may descend on an instrument approach without the required visual references. DA (Decision Altitude) applies to precision and approach-with-vertical-guidance procedures: at this altitude on the descent, the pilot must either continue visually or execute a missed approach. MDA (Minimum Descent Altitude) applies to non-precision approaches: the pilot may descend to and level off at this altitude, but must not go below it unless the required visual references are in sight and a normal landing can be made.
Plain English
The lowest altitude you are allowed to fly down to on an instrument approach. If you can see the runway environment by then, you can keep going down and land. If you can't, you have to go around.
Context Anchor
Seen in the minimums section of helicopter GPS approach charts for an airport or heliport.
Derivation
Decision Altitude is named for what happens there: the pilot makes a go/no-go decision. Minimum Descent Altitude is named for what it sets: the minimum altitude you may descend to. The naming difference matches a real operational difference -- DA is a point you pass through (briefly) while deciding; MDA is a floor you level off at and hold.
Why Pilots Care
Marks the exact point where continuing without visual contact becomes unsafe and a missed approach must be initiated.
Intuition Check
DA and MDA are not the same kind of minimum. DA is a decision point while descending; MDA is a floor you may not go below until you can continue safely to land.
Example Sentence 1
Approaching the DA, the pilot called the runway environment in sight and continued the approach to landing.
Example Sentence 2
The helicopter leveled at the MDA until the runway environment came into view, then descended to land.