Crimson is a strong, bright, deep red color. It originally meant the color of the Kermes dye produced from a scale insect, Kermes vermilio, but the name is now also used as a generic term for slightly reddish-blue colors that are between red and rose; besides crimson itself, these colors include carmine, raspberry, ruddy, ruby, amaranth, and cerise. The color was very common for casual clothing in Ancient Roman culture. The Romans called the color purple.

The word crimson has been recorded in English since 1400, and its earlier forms include cremesin, crymysyn and cramoysin (cf. cramoisy, a crimson cloth). These were adapted via Old Spanish from the Medieval Latin cremesinus (also kermesinus or carmesinus), the dye produced from Kermes scale insects, and can be traced back to Arabic qermez ("red"), also borrowed in Turkish and many other languages, e.g. German Karmesin, Italian Cremisi, French cramoisi, etc. (via Latin). The ultimate source may be Sanskrit "kr.mi-ja" meaning "worm-made."