Depending on the situation, "morning" can either by any time after midnight and before midday, or any time after sunrise and before midday. I've never heard of any of the other three words before. "Forenoon" sounds like someone replaced the "after" in "afternoon" with "fore" (meaning "before") to get something that means "before noon". Apparently it's a North American nautical term: http://www.oxforddictionaries.com/definition/english/forenoon According to Google, "Ante meridiem" isn't English, it's Latin for "before noon". I just realised that this is what "am" stands for, but I've never heard the abbreviated version before in my whole life. I've also never heard "ack emma". I googled it and it's apparently an archaic, informal and British way of saying "am": http://www.oxforddictionaries.com/definition/english/ack-emma