Ecliptic
The Sun's apparent path around the sky over a year, which is really Earth's orbit seen from inside. The zodiac is a band around it, and every chart position is measured along it.
Watch the Sun against the background stars for a year and it traces one great circle around the sky. That circle is the ecliptic, and it is Earth's own orbital plane projected outward. The planets orbit near that same plane, so they keep to a narrow band around it; the Moon strays about five degrees to either side.
The ecliptic is the spine of the whole system. The zodiac is the band centered on it, the signs are twelve equal segments of it, and a position in any chart, natal or current, is a longitude measured along it. When a chart says Mars at 14 degrees Leo, that is an address on the ecliptic.
The name is the giveaway: it is where eclipses happen. The Moon crossing the ecliptic at its nodes, during a New or Full Moon, is the entire recipe.