Ecliptic
The Sun's apparent yearly path across the sky — the line the entire zodiac is measured along, in degrees, all the way around back to where it started.
From Earth, the Sun appears to trace the same path across the background stars every year, a great circle called the ecliptic. The whole zodiac is laid out along this one line, split into twelve 30-degree stretches, one per sign, starting from 0 degrees Aries at the spring equinox. Every planet in regular use stays close to this same band, within a few degrees either side, and that shared corridor is why the classical planets all seem to travel roughly the same route across the sky rather than scattering in every direction. A planet's zodiac sign, and its exact degree within that sign, is really just its position measured along this one line at a given moment — the ecliptic is the ruler every other measurement in a chart is taken against.