- or AMabbreviation for the Latin ante meridiem, "before noon," used after a time to indicate that the time is before 12:00 (noon). The notations "am" and "pm" are used extensively in the United States, Canada, and other countries where time is usually not stated on a 24-hour basis. By convention, midnight is represented as 12:00 am and noon as 12:00 pm. In text, however, it is best to avoid the use of 12:00 am or 12:00 pm since the reader may not be aware of these conventions.
Dictionary of units of measurement. 2015.