Eighteenth \Eight"eenth`\, n.1. The quotient of a unit divided by eighteen; one of eighteen equal parts or divisions.
2. The eighth after the tenth.
Source: Webster's Revised Unabridged Dictionary (1913)
Eighteenth \Eight"eenth`\, a. [From Eighteen.]1. Next in order after the seventeenth.
2. Consisting of one of eighteen equal parts or divisions of a thing.