undĕcĭm, num. adj. [unus-decem],
eleven : legiones, Cic. Fam. 6, 18, 2: milia debeo, Mart. 2, 44, 8; Vitr. 3, 1 med. : dies, Macr. S. 1, 13.
ūndecim or XI num. unus+decem, eleven : XI legiones: milia talentum. ...
An Elementary Latin Dictionary