The basic legal standard used to decide the constitutionality of nearly all federal and state laws, policies, or practices.
Under the rational basis test, the burden is on the party challenging a law, policy, or practice to show that it isn't rationally related to a legitimate government interest. This test is exceptionally deferential to the government, and extremely difficult for a challenger to satisfy. As a general rule, the rational basis test is applied to all laws, policies, and practices that don't involve suspect classifications or fundamental rights (which get strict scrutiny) or quasi-suspect classifications (which get intermediate scrutiny).
For an example of the rational basis test, see City of New Orleans v. Dukes, 427 U.S. 297 (1976) (ordinance banning street vendors but grandfathering in certain vendors satisfied rational basis test).