Cornell University defines a legacy applicant as a student whose parents, grandparents, or earlier ancestors hold a degree from Cornell University (including undergraduate, graduate, and professional programs).
(I’m not sure if this italicized statement is 100% factual.)
Below is a Q&A directly from the Cornell webpage:
Here is also a link to an old CC thread covering Cornell legacy.
Legacy students (children of Cornell undergraduate alumni) typically make up about 14.7% to 22.1% of an incoming class depending on the cohort (for instance, legacy students comprised 14.7% of the Class of 2025 and 22.1% of the Class of 2024 Early Decision pool). It would make sense for Cornell to put more weight on legacy during ED than RD.