1Cloth of (or trimmed with) a rich crimson or purple colour; specifically (with the) a robe or garment made of this, worn as the distinguishing dress of Roman and Byzantine emperors. Hence figurative: imperial office, rank, or power.
2Any of various rich crimson or purple colours. Now especially: a deep, rich violet-purple; a shade or tint of this.
3Any of various dyes or pigments used to produce one of these colours; especially those extracted from certain molluscs of the family Muricidae (also known as Tyrian purple) (now historical).
(A) Of clothing, etc.: made of imperial purple; (also figurative) royal, imperial, exalted. (b) Of a deep crimson or violet-purple colour.
Late Middle English; earliest use found in Osbern Bokenham (c1393–c1464), poet and Augustinian friar. In some forms from purple + imperial.