What is a flat bean in coffee?

A flat bean is a standard coffee bean - the term exists specifically to distinguish it from a peaberry. Most coffee cherries contain two seeds that develop pressed against each other, giving each a characteristically flat inner face. Those are flat beans.

The term comes up most often on East African green coffee specifications, where the grade designation includes physical form. A Kenyan AA flat tells you the lot has been sorted to remove peaberries, leaving only the standard flat beans of AA screen size.

The practical relevance for roasting: flat beans and peaberries behave somewhat differently in the drum - peaberries' rounded shape means they tumble more freely and can develop at a slightly different rate. Whether separated peaberries produce a meaningfully better cup is debated, but the distinction is real enough that it appears on specifications and is worth understanding when it does.