The reference material that I find most helpful in define a new word is

a dictionary. It provides the most accurate and comprehensive definition of a word, along with additional information such as pronunciation, synonyms, antonyms, and usage examples. Online dictionaries also often include audio pronunciations and allow users to hear how the word is pronounced. Additionally, some dictionaries provide the etymology and history of a word, which can be helpful in understanding its origins and development over time. Overall, a dictionary is a reliable and essential reference material for defining new words.