Given the policies of Pres. Roosevelt in the time between the start of WWII in Europe and the Pearl Harbor attack in December of 1941, the best way to describe American neutrality is to say that it was only skin deep. In other words, America was neutral in name only.
In truth, the US was taking the Allies' side. The US took many steps to aid the Allies. The most blatantly non-neutral of these was the US policy (started in the spring of 1941) of escorting convoys across the Atlantic. The US Navy did this to allow merchant ships to have a better chance of getting to England carrying supplies from the US. The US Navy would alert the British navy to any German ships they saw. By doing these sorts of things, the US was clearly taking the Allies' side.
Because FDR took this and other actions (such as Lend-Lease) it is clear that American neutrality was not sincere. In truth, the US was doing all it could to help the Allies short of actually joining the war.