What definition best describes the term downstage?

Downstage is the front of the stage, the part nearest the audience. The best way to remember this is to remember that back in the day, stages used to be raked or slanted toward the front, which gave audience members whose heads were at the level of the front of the stage a better view of what was going on further upstage. At that time upstage was literally higher than downstage.

The expression "to upstage someone" means to take their limelight, to push yourself into something where they are supposed to more important. This comes from actors coming between another actor and the audience thus blocking out the upstage actor.