Plain English Meaning
The lowest altitude you are allowed to fly down to on a non-precision approach when you are lined up with the runway and not circling to land. You stay at or above this altitude until you can see the runway environment well enough to land.
Full Definition
The Minimum Descent Altitude published for a straight-in (non-circling) instrument approach. It is the lowest altitude, expressed in feet MSL, to which a pilot may descend on the final approach segment of a non-precision approach when the runway alignment allows landing without a circling maneuver.
Context Anchor
Seen in the minimums section of an and in - altitude discussions.