na-adjective, noun which may take the genitive case particle 'no', noun
great; huge; enormous; serious
Used for large quantities, amounts, or degrees, often in formal or written contexts. Can modify nouns directly (多大な/多大の) or be used as a noun.
多大なご支援をいただき、ありがとうございます。
Thank you for your great support.
その事故は多大の損害をもたらした。
The accident caused enormous damage.