Definition of down:
(v) :
from a higher position to a
lower one;
downwards;
South (as south is at the bottom of typical maps)
"the cat jumped down from the table."
(preposition) :
from one end to another; especially, from a higher end to a lower
"the ball rolled down the hill."
(adj) :
Depressed, feeling
low; on a lower level than before
"so, things got you down? / Is Rodney Dangerfield giving you no respect? / Well, bunky, cheer up!"
"I bet after the third down, the kicker will replace the quarterback on the field."