thirtieth(n = noun.linkdef) - position 30 in a countable series of things;
is a kind of rank
thirtieth(s = adj.all) 30th - coming next after the twenty-ninth in position;
1. Next in order after the twenty-ninth; the tenth after the twentieth; -- the ordinal of thirty; as, the thirtieth day of the month. [1913 Webster]
2. Constituting or being one of thirty equal parts into which anything is divided. [1913 Webster]
The quotient of a unit divided by thirty; one of thirty equal parts. [1913 Webster]