The act of laying down on something, and flailing your arms and legs about to music, thus creating the effect that you are dancing.
(Laying on bed talking)
Kenzie: This is a good song!
Caitlin: I have to dance but im too lazy to get up...
Kenzie: Now is a time for Horizontal dancing!
Gettin' jiggy with it
Barry Whitesque style term referring to the act of indulging in a bit of hide the sausage
"Let's turn the lights down low baby, have a little wine and later... do a little horizontal dancing"