1) Being one of 90 equal parts 2) Next after the eighty-ninth 3) Notable anniversary 4) Notable birthday 5) Ordinal 
• (a.) Next in order after the eighty-ninth. • (n.) The quotient of a unit divided by ninety; one of ninety equal parts of anything. • (n.) The next in order after the eighty-ninth. • (a.) Constituting or being one of ninety equal parts. • (a.) Next in order after the eighty-ninth. • (a.) Constituting or being one of n...
Nine'ti·eth adjective 1. Next in order after the eighty-ninth. 2. Constituting or being one of ninety equal parts. 
[adj] - the ordinal number of ninety in counting order 2. [n] - position 90 in a countable series of things
90th adjective the ordinal number of ninety in counting order
next after the eighty-ninth; being the ordinal number for 90. · being one of 90 equal parts.