In the English language, the expressions “a.m.” and “p.m.” are both commonly used to differentiate morning versus evening times of the day. This distinction is not necessary in many parts of the world where a 24-hour clock is used, or when “in the morning’ or “in the afternoon” are specified as such. Instead, “a.m.” and “p.m.” are incredibly useful for the 12-hour clock, helping differentiate vastly different hours of the day, such as 4 a.m. versus 4 p.m. Yet the question remains: What do “a.m.” and “p.m.” actually stand for?
As with a lot of the English language, Latin can be credited with the use of these expressions in modern speech. “A.m.” actually stands for “ante meridiem” and “post meridiem”. “Meridiem” refers to the middle of the “diem”, or “day”, which is of course viewed as noon. Even on a sundial, as used by the ancient Romans, the midpoint of the dial can be found at this time. “Ante”, therefore means “before”, and “post” means “after”. When someone defines a time as being “ante meridiem”, he or she is literally stating that the time falls earlier in the day than when the sun is at its highest and casts a noon shadow on a sundial clock. Likewise, a time labeled as “post meridiem” is an hour that falls after the sun has held the noon position on the sundial.
There are many expressions in the English language that are embedded with Latin. The “a.m.”-“p.m.” phrase origin is just one example.