Definition of down:
from a higher position to a lower
(as south is at the bottom of typical maps)
"the cat jumped down from the table."
from one end to another; especially, from a higher end to a lower
"the ball rolled down the hill."
, 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."