n.

1. 落下；影响；落下（或影响）方式[U][S1]
2. 发生率；影响范围[U][S1]
   There's a high incidence of heart disease there.

   那个地方心脏病的发病率很高。

3. 【物】射入；入射角
4. 【数】关联；接合

n.

1. the occurrence, rate, or frequency of a disease, crime, or other undesirable thing.
2. Physics  the intersection of a line, or something moving in a straight line, such as a beam of light, with a surface.
3. the way in which the burden of a tax falls upon the population.