Definitions of twenty-five. noun. the cardinal number that is the sum of twenty-four and one. synonyms: 25, XXV. see more. adjective. being five more than twenty. synonyms: 25, xxv. cardinal. being or denoting a numerical quantity but not order. Pronunciation. US. /twɛniˈfaɪv/ UK. /twɛntiˈfaɪv/ Cite this entry. Style: MLA. "Twenty-five." iRd6.