Declination
In plain words
How far north or south of the celestial equator a planet is. The chart wheel only shows the sideways position; this is the up-and-down one.
In the tradition's terms
Angular distance from the celestial equator, the complement to the longitude the wheel is drawn in.
In depth
Bodies at the same declination are in parallel; at equal and opposite declinations, contraparallel. Read much as a conjunction and an opposition respectively, and invisible on a conventional wheel.