Commonly refers to the upright version of a face within a font family, as compared to the italic version.
Adobe Glossary of typographic terms
In Macintosh font menus, this is called Plain meaning text that has no style applied to it (i.e., Italic, Bold, Boldltalic). Roman fonts are upright thick-and-thin weighted, and usually serifed type. The classical Roman letter style began in A.D. 114 with letters chiseled in the stone of the Trajan Columns in Rome.
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The (standard) upright type style. The term Roman is also sometimes used to denote the Regular weight. See Styles, Weights, Widths - It’s All in the (Type) Family on The FontFeed.
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