Google successfully made its case to a jury last year that its use of Java APIs in Android was "fair use." A San Francisco federal jury rejected Oracle's claim that the mobile system infringed Oracle's copyrights.

But Oracle isn't backing down. Late Friday, the company appealed the high-profile verdict to a federal appeals court.

This is the latest stage of a seemingly never-ending legal battle over intellectual property that began in 2010. The conflict has meandered through two federal trials, in addition to multiple trips to the appellate courts and to the Supreme Court.

Oracle opened its brief to the US Court of Appeals for the Federal Circuit right where it left off after losing its case. Among other things, Oracle is refusing to believe that the "fair use" defense to copyright-infringement allegations should have protected Google from having to pay billions of dollars in damages.

"When a plagiarist takes the most recognizable portions of a novel and adapts them into a film, the plagiarist commits the 'classic' unfair use," Oracle said in its opening brief.

Google's copying in this case is the software equivalent of this classic unfair use. Google copied thousands of lines of copyrighted code from Oracle's Java programming platform. Google concedes it put that code to the same use in the competing Android platform, for what this Court already has deemed "entirely commercial" purposes. And Google reaped billions of dollars while leaving Oracle's Java business in tatters.

Fair use is a defense to copyright infringement if certain elements are met. It's decided on a case-by-case basis. "There is no specific number of words, lines, or notes that may safely be taken without permission," according to the US Copyright Office. There are, however, at least four factors to be considered when deciding fair use: the purpose of use, the nature of the copyrighted work, the amount and substantiality of the portion taken, and the effect of the use upon the potential market.

Before going to the appeals court, Oracle asked US District Judge William Alsup to overturn the jury's verdict. Alsup, who presided over the second trial, ruled that Google's use cleared all four factors.

Here's how we got to this point:

Oracle purchased Sun Microsystems and acquired the rights to Java in 2009. Oracle then sued Google in 2010, saying that Google infringed copyrights and patents connected to Java. The case went to federal trial in 2012. Oracle initially lost. But part of its case was revived on appeal and another trial was ordered. The sole issue in the second trial, the one now being appealed, was whether Google infringed the APIs in Java, which the appeals court held were copyrighted. In May, a jury found in Google's favor after the second trial. The jury found that Google's use of the APIs was protected by "fair use"—a decision Alsup refused to disturb.

Google declined to comment on the appeal. Google must file its response in the coming months.