A guarantee under the 14th Amendment that a state must treat an individual or a class of individuals (such African-Americans, women or the disabled) the same as it treats other individuals and classes in the same circumstances. The equal protection clause of the Constitution protects against laws that affect people differently without a rational basis for doing so. In reviewing claims of denial of equal protection, a court will uphold legislation that has a rational basis unless the law affects a fundamental right or involves a class of persons that the court finds needs special protection due to a history of discrimination against them.
For example, the court has found that voting is a fundamental right and that race, national origin and religion are suspect classifications deserving additional protections. In those instances, a court will use a strict scrutiny standard of review and will strike down the legislation unless the government can show a compelling need for the discrimination.
Justice Learning, The United States Constitution, what it says, what it means, A Hip Pocket Guide (Oxford University Press)