The state of being null or void; nothingness. The contract was declared a nullity by the court.
A thing of no importance or worth. His opinion was considered a nullity in the decision-making process.
A legal term referring to something that is invalid or without legal force. The marriage was annulled and treated as a nullity.
Mathematics: the dimension of the null space of a matrix or linear transformation; the number of solutions to the homogeneous equation A x = 0. Because the matrix has rank three and four columns, its nullity is one.