The first body that can sit anywhere in a chart.
Mars is the first planet not tethered to the Sun's position. Mercury and Venus are always nearby; from Mars outward, a planet can be at any angle to the Sun, which is what makes oppositions to the Sun possible at all.
It ruled Scorpio as well as Aries for two thousand years, until Pluto's discovery in 1930 reassigned it. Both schemes remain in use, and a Scorpio chart reads differently depending which you take.
Its retrograde periods are long and pronounced — at opposition Mars is also at its brightest and closest, which is why the retrograde loops of Mars were the observations that eventually broke the geocentric model.
What it is
The first planet outside Earth's orbit.