nullity

From Middle French nullité, from Medieval Latin nullitas, from Latin nullus.

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nullity (countable and uncountable, plural nullities)

1. The state of being null, or void, or invalid.

   nullity of marriage

2. (law) A void act; a defective proceeding or one expressly declared by statute to be a nullity.
3. (mathematics) The difference between the rank of a matrix and the number of columns it has; the dimension of the nullspace of a matrix.

• decree of nullity
• rank-nullity theorem