A team of Nazi supporters during World War II, composed of costumed crime-fighters who chose to side with the German war machine against the Allied Powers for various reasons.
The Battle-Axis were actual comic heroes from the 1940's, but not from Marvel / Timely Comics. Instead they were characters who had fallen into public domain after their original publishers went out of business.
Other members unrelated to the X-Men include the Human Meteor, Spider-Queen, Volton and Strongman.