decimal:  Decimal comes from decimālis, a medieval Latin coinage based on decimus ‘tenth’, the ordinal derivative of Latin decem ‘ten’ (a relative of English ten). Other descendants of decem include December, decimate, dime, and the prefix deci-, while its Greek cousin déka has given us decade  and the prefix deca-. => decade, december, decimate, dime, ten, tithe
c. 1600, from Medeival Latin decimalis "of tithes or tenths," from Latin decimus "tenth," from decem "ten" (see ten). Applied to Arabic notation before modern sense of "decimal fractions" emerged. As a noun from 1640s.