One-person, one-vote refers to the rule that one person’s voting power ought to be roughly equivalent to another person’s within the same state.
The rule comes up in the context of strategically drafting voting laws and gerrymandering, used to increase the voting power of particular groups like rural voters to the disadvantage of other groups. Equal Protection as guaranteed by the 5th and 14th amendments require broadly that each person be treated equally in their voting power, but what equality means relies on a series of Supreme Court cases. The most relevant Supreme Court case is Reynolds v. Sims, 377 U.S. 533 (1964). In that case, the Court held that states need to redistrict in order to have state legislative districts with roughly equal populations: "The Equal Protection Clause requires substantially equal legislative representation for all citizens in a State regardless of where they reside." In Evenwel v. Abbott, 578 U.S. __ (2016), the Supreme Court held that when drawing legislative districts, state legislatures may use the total population of areas within the state, rather than being restricted to using the voting-eligible populations.
For more on the one-person, one-vote rule, see this University of Florida Law Review article, this University of Michigan Law Review article, and this article in The Atlantic.
[Last updated in May of 2022 by the Wex Definitions Team]