imaginary

The story

1. Latin · derived fromimāginārius“relating to images, fancied”
2. Middle English · inherited fromymaginarie
3. English todayimaginary

From Middle English ymaginarie, ymagynary, from Latin imāginārius (“relating to images, fancied”), from imāgō, equivalent to imagine + -ary. The mathematical sense derives from René Descartes's use (of the French imaginaire) in 1637, La Geometrie, to ridicule the notion of regarding non-real roots of polynomials as numbers. Although Descartes' usage was derogatory, the designation stuck even after the concept gained acceptance in the 18th century.

First known use, by meaning

• from 16th c. Imagination; fancy.
• from 18th c. An imaginary number.
• from c. 1975 The set of values, institutions, laws, and symbols common to a particular social group and the corresponding society through which people imagine their social whole.