Definition
The ecliptic is the apparent path the Sun traces against the background stars over the course of a year. It is essentially the plane of Earth's orbit projected onto the sky, seen from our moving vantage point.
In context
Because the planets orbit the Sun in roughly the same plane as Earth, they — along with the Moon — are always seen close to the ecliptic. The constellations the Sun passes through over the year lie along this same band.
Related ideas
The ecliptic is tilted relative to Earth's equator, a tilt that gives rise to the seasons. The band of constellations centered on the ecliptic is known as the zodiac.