Noun
A key; a glossary.
Source: Webster's dictionaryClavis Books is generously giving away a copy of Click this link to enter the Rafflecopter giveaway. Source: Internet
William Oughtred used and δ, the Greek letter equivalents of p and d, to express ratios of periphery and diameter in the 1647 and later editions of Clavis Mathematicae. Source: Internet