AM definition: 1. I form of be: 2. used when referring to a time between twelve o'clock at night and twelve…. Learn more. In particular, AM stands for ante meridiem, which translates into English as ‘before midday’. Meanwhile, PM is post meridiem and means in English ‘after midday’. These abbreviations are used to identify the time in the 12-hour clock format instead of using 24 hours. Allied Mastercomputer (also called Adaptive Manipulator or Aggressive Menace ), or AM, the antagonist in the short story and video game I Have No Mouth, and I Must Scream by American writer Harlan Ellison Originally there were two cycles: one cycle which could be tracked by the position of the Sun (day), followed by one cycle which could be tracked by the Moon and stars (night). This eventually evolved into the two 12-hour periods which are used today, one called a.m. starting at midnight and another called p.m. starting at noon. [1] The 24-hour clock is a method of telling the time in which the civil day runs from midnight until next midnight. The following numbers (used once in the same civil day) of hours are: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22 and 23, without AM nor PM. AM and PM are the abbreviated terms for Ante Meridiem and Post Meridiem respectively. While AM represents the time from midnight to 11:59 noon, and PM represents the time from 12 noon to 11:59 midnight. AM means before noon. This is the 12-hour period from midnight to noon. PM means after noon. It covers the 12 hours from noon to midnight. Using numbers from 1 to 12, followed by am or pm, the 12-hour clock system identifies all 24 hours of the day. The meaning of AM is present tense first-person singular of be. Learn what is am and pm in the 12-hour clock system. AM means before midday and PM means after. Master time telling with simple rules and examples. AM covers the period from midnight up to noon. PM covers the period from noon to the next midnight. The 12-hour clock divides the day into two cycles of 12 hours each. Instead of counting continuously from 0 to 23, it repeats the range from 1 to 12.