The story
After Lemon v. Kurtzman, the United States Supreme Court case in which the test was first spelled out. Lemon is the surname of its lead plaintiff, Alton Lemon.
Etymology from Wiktionary, CC BY-SA 4.0.
Lemon test
Today it means: The test for whether a government action violates the Establishment Clause of the United States' constitution: it does so if it lacks a secular purpose, has its primary effect as promoting or inhibiting religion, or fosters an excessive entanglement of government with religion. Full definition →