The Freedom From Religion Foundation article says:
[The Lemon Test] also codified existing precedent on the Establishment Clause into a test, which is called the "Lemon Test." You can probably recite the "Lemon Test" with us. It has three prongs and if any of the three prongs are violated by an act of government, it is unconstitutional:
1. It must have a secular legislative purpose.
2. Its principal or primary effect must neither advance nor inhibit religion.
3. It must not foster excessive entanglement between government and religion.
This was not new law, per se, but a kind of noble attempt to clarify and make the Establishment Clause idiot-proof. The "Lemon Test" has been invoked in virtually every lawsuit FFRF has ever taken. It is our best friend.
This is one of those cases where we're reminded we should know names like this much better than those of actors or athletes. I am proud to learn that Lemon was from my native Pennsylvania, although I *still* didn't know this. Shame on me!