a.m.

1. 1
   adverb

   During the first half of the day, from midnight to noon, when telling time on a 12-hour clock.

   The bakery opens at 5 a.m. to start the day's bread.

   Her flight was delayed until almost 2 a.m. by the storm.

   Note

   Two Latin words sit behind these letters: ante meridiem, "before noon." The abbreviation always follows the hour (9 a.m., never a.m. 9) and may be set in capitals, as A.M. Twelve o'clock is the trap. Some writers use 12 a.m. for midnight and 12 p.m. for noon, while others reject both and write 12 midnight or 12 noon, which no reader can misread.

2. 2
   adjectiveinitialism

   Initialism of ad modum.

   Note

   A Latin phrase reduced to its initials: ad modum, meaning "consistent with." It is spelled exactly like the clock abbreviation, but it never follows a number. Instead it comes before the model or authority that something conforms to, as in a.m. the Roman model. If the letters follow an hour, they mean morning. If they introduce a pattern to follow, they mean this.

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