ar·i·ty (âr??-t?)n. pl. ar·i·ties The number of arguments or operands taken by a function or operator.[-ar(y) (as in binary ternary) + -ity.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.