In typography, a serif (/ˈsɛrɪf/) is a small line or stroke regularly attached to the end of a larger stroke in a letter or symbol within a particular font or family of fonts. A typeface or "font family" making use of serifs is called a serif typeface (or serifed typeface), and a typeface that does not include them is sans-serif. Some typography sources refer to sans-serif typefaces as "grotesque" (in German, grotesk) or "Gothic" (although this often refers to blackletter type as well). In German usage, the term Antiqua is used more broadly for serif types.

Serif typefaces can be broadly classified into one of four subgroups: Old-style, Transitional, Didone, and Slab serif, in order of first emergence.

Serifs originated from the first official Greek writings on stone and in Latin alphabet with inscriptional lettering—words carved into stone in Roman antiquity. The explanation proposed by Father Edward Catich in his 1968 book The Origin of the Serif is now broadly but not universally accepted: the Roman letter outlines were first painted onto stone, and the stone carvers followed the brush marks, which flared at stroke ends and corners, creating serifs. Another theory is that serifs were devised to neaten the ends of lines as they were chiselled into stone.

The origin of the word 'serif' is obscure, but apparently is almost as recent as the type style. The book The British Standard of the Capital Letters Contained in the Roman Alphabet, Forming a Complete Code of Systematic Rules for a Mathematical Construction and Accurate Formation of the Same, with Letters of Exemplification, Elementary and Compleat: Together with an Useful and Practical Appendix by William Hollins (1813), defined 'surripses', usually pronounced "surriphs", as "projections which appear at the tops and bottoms of some letters, the O and Q excepted, at the beginning or end, and sometimes at each, of all". The standard also proposed that 'surripsis' may be a Greek word derived from σῠν- ('syn-', "together") and ῥῖψῐς ('rhîpsis', "projection").

In 1827, Greek scholar Julian Hibbert printed with his own experimental uncial Greek types, remarking that the types of Giambattista Bodoni's Callimachus were "ornamented (or rather disfigured) by additions of what [he] believe[s] type-founders call syrifs or cerefs". The printer Thomas Curson Hansard referred to them as "ceriphs" in 1825. The oldest citations in the Oxford English Dictionary (OED) are 1830 for 'serif' and 1841 for 'sans serif'. The OED speculates that 'serif' was a back-formation from 'sanserif'.

Webster's Third New International Dictionary traces 'serif' to the Dutch noun schreef, meaning "line, stroke of the pen", related to the verb schrappen, "to delete, strike through" ('schreef' now also means "serif" in Dutch). Yet, schreef is the past tense of schrijven (to write). The relation between schreef and schrappen is documented by Van Veen and Van der Sijs. In her book Chronologisch Woordenboek, Van der Sijs lists words by first known publication in the language area that is the Netherlands today:

The OED's earliest citation for "grotesque" in this sense is 1875, giving 'stone-letter' as a synonym. It would seem to mean "out of the ordinary" in this usage, as in art 'grotesque' usually means "elaborately decorated". Other synonyms include "Doric" and "Gothic", commonly used for Japanese Gothic typefaces.

Old-style typefaces date back to 1465, shortly after Johannes Gutenberg's adoption of the movable type printing press. Early printers in Italy created types that broke with Gutenberg's blackletter printing, creating upright ("roman") and then oblique ("italic") styles that were inspired by Renaissance calligraphy. Old-style serif fonts have remained popular for setting body text because of their organic appearance and excellent readability on rough book paper. The increasing interest in early printing during the late 19th and early 20th centuries saw a return to the designs of Renaissance printers and type-founders, many of whose names and designs are still used today.