vi·ges·i·mal 
(vĭ-jĕsə-məl)
(vĭ-jĕsə-məl)adj.
1. Twentieth.
2. Proceeding or occurring in intervals of 20.
3. Based on or relating to 20.
[From Latin vīgēsimus, variant of vīcēsimus, twentieth, from vīgintī, twenty; see wīktī- in the Appendix of Indo-European roots.]