1.S 回归线either of the two circles round the earth, one about 23½ degrees south of the equator
2.P 热带，热带地方　the parts of the world between these two lines

　in the ～s　在热带地区
　the T- of Cancer〔Capricorn〕 　北〔南〕回归线

　Darkness falls quickly in the tropics.　热带地区天黑得很快。
　I went to the tropics for the first time last year.　我去年第一次去了热带地区。
　The coconut is particular to the tropics.　椰子是热带地区的特产。