The intersection of the plane of the Earth's orbit with the celestial sphere.
The great circle in the celestial sphere that is the apparent annual path of the sun around the earth.
The plane in which this circle lies is the plane of the ecliptic. The angle that this plane makes with the plane passing through the earth's equator is the obliquity of the ecliptic. With reference to the actual motion within the solar system, the plane of the earth's orbit is equivalent to the plane of the ecliptic. That is, the intersection of the plane of the earth's orbit with the earth's surface is the same as the projection of the ecliptic upon the earth's surface. The most northern and southern points on this line of intersection define the latitudes of the Tropic of Cancer and Tropic of Capricorn, respectively.
Ecliptic has been defined 2 different ways in documents like Tide and Current Glossary, Glossary of Meteorological Terms.