1) A circle on the celestial sphere which runs parallel to the celestial equator at an angle of 23 degrees and 26 minutes. There are two tropics: a northern and a southern one. In the northern hemisphere the sun reaches the northern tropic of Cancer at the summer solstice around the 22nd June. At one point in time this coincided with the Sun standing in the constellation of Cancer, although due to the precession, the summer solstice now occurs against the backdrop of the constellation of Gemini. After reaching the tropic of Cancer, the Sun (apparently) moves in the direction of the celestial equator. It reaches the tropic of Capricorn at the winter solstice around the 22nd December which, owing to the procession, takes place against the backdrop of the constellations of Sagittarius and not against the backdrop of the constellation Capricorn after which it was named.
2) The equivalent circle on the Earth's surface which runs parallel to the equator - also at an angle of 23 degrees 26 minutes. The summer solstice occurs when the Sun reaches its zenith at the northern tropic of Cancer and then moves southwards to reach the southern tropic of Capricorn, which marks the time of the winter solstice.