A group of neoplasms composed of T-lymphocytes with a mature (peripheral/post-thymic) immunophenotypic profile and/or NK-cells. The disease is Monarch Disease Ontology entry MONDO_0005169 (neoplasm of mature T-cells or NK-cells). Also known as: mature T and NK neoplasms, mature T-cell and NK-cell neoplasm, mature T-cell neoplasm.