Suspect classification refers to a class of individuals that have been historically subject to discrimination.
Overview
Under Equal Protection, when a statute discriminates against an individual based on a suspect classification, that statute will be subject to either strict scrutiny or intermediate scrutiny.
There are four generally agreed-upon suspect classifications: race, religion, national origin, and alienage. However, this is not an inclusive list.
Determining a Suspect Classification
"Discrete and insular minorities"
In determining whether someone deserves to be considered within a suspect classification, a court will look at whether the person is a "discrete and insular minorit[y]." In determining whether someone is a discrete and insular minority (and thus the person's claim deserves strict scrutiny), courts will look at a variety of factors, including but not limited to whether the person has an inherent trait, whether the person has a trait that is highly visible, whether the person is part of a class which has been disadvantaged historically, and whether the person is part of a group that has historically lacked effective representation in the political process.
Further Reading
For more on suspect classifications, see this Seattle University Law Review article and this St. John's Law Review article.