According to the Associated PressStyle time of day, the time descriptors "ante meridiem" (before noon) and "post meridiem" (after noon) should be shortened with lowercase letters and periods (a.m. and p.m.). Ante means before in Latin, and ante meridiem is used to indicate that a statement applies before noon. Post means after in Latin, and post meridiem is used to indicate that a statement applies after noon. Thus, ante meridiem and post meridiem are the only correct ways to write this phrase. The word meridiem comes from the Latin words for midday, which are mero diem. Thus, ante meridiem and post meridiem mean before midday and after midday, respectively. In English, we often omit the word late when referring to afternoon hours, so "in the afternoon" can be written as simply "pm". But in Latin, the word is late, which makes these phrases correct: ante meridianum and post meridianum. Also note that meridianum is an adjective meaning "about midday". So, these other phrases are also correct: ante meridie and post meridie.