day one

noun, often capitalized D&O

Definition of day one

  1. :  the first day or very beginning of something

Examples of day one in a Sentence

  1. We've known this about the project since Day One.

  2. since day one of this project, we've always insisted that we would create the best product possible without any consideration of cost

First Known Use of day one

1971


DAY ONE Defined for English Language Learners

day one

noun

Definition of day one for English Language Learners

  • : the first day or very beginning of something


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