directional, punctuation, noun · the lowest part of something

他潜到了水底。

directional · the end of a period of time, such as a month or a year

我们打算在今年六月底全家去海边旅游。

noun, verb · Equivalent to 的 de as a possessive particle.

这个水坑很深，一眼望不到底。

directional · background color, foundation, or the base level of something

这块布是蓝底红花的。

noun, directional · a base or radix in mathematics

在对数运算中，我们通常把十作为底。

directional · a draft, copy, or original manuscript kept as a record

发信前请留个底。