AM、PM是上午和下午的英文缩写，请问它们的完整单词是什么？

This is incorrect:

AM：after midnight
PM：prior to midnight

It is:

AM: ante meridiem (before noon)
PM: post meridiem. (after noon)

These words come from Latin (拉丁语)

ante = prior to, before
post = after
meridiem = noon

note: 12 am = midnight, 12 pm = noon
However it is often better to use midnight or noon to be clear in this case.

In my high school English textbook it said that you could write 12 m for noon, however I have never seen it used.

ante meridiem [拉]午前, 上午
下午；午后 (post meridiem)

after midnight （拉丁语ante meridiem）
prior to midnight （拉丁语post meridiem）

楼上说的很对！根本不是after midnight 什么的！

成功领路
-
经理
四级
11-8
07:39
a.m.=ante
meridiem=before
noon
上午
p.m.=post
meridiem(=afternoon)下午
此外
b.c.--before
christ
公元前...年(基督之前...年)
a.d.--=anno
domini
(=in
the
year
of
our
lord)
【拉】公元...年(原义为主的纪年)
考虑的很周到