class.

/K L AE1 S/
noun
  1. 1

    The act of forming into a class or classes; a distribution into groups, as classes, orders, families, etc., according to some common relations or attributes.

verb
  1. 1

    To identify by or divide into classes; to categorize

    Should we classify "make up" as an idiom or as a phrasal verb?

  2. 2

    To declare something a secret, especially a government secret

    They decided to classify that information.

Antonyms

Source: https://en.wiktionary.org/wiki/class., https://en.wiktionary.org/wiki/classification, https://en.wiktionary.org/wiki/classify