This term was coined by Thaler and Sunstein (2008) and refers to the practice of influencing choice by changing the manner in which options are presented to people. For example, this can be done by setting defaults, framing, or adding decoy options.
Thaler, R. H., & Sunstein, C. (2008). Nudge: Improving decisions about health, wealth, and happiness. New Haven, CT: Yale University Press.