TALLAHASSEE, Fla. -- Jurors, perplexed by an undefined legal term, were unable to reach a verdict Monday in the hazing trial of five Florida A&M University fraternity members and the judge declared a mistrial.

The trial would have been the first to test a new state law that makes hazing a felony if it results in death or "serious bodily injury," but the law does not define the latter term.

The mistrial was declared about 20 minutes after the six-member jury sent a note to Circuit Judge Kathleen Dekker asking for a more substantial definition. The jurors also asked how to distinguish between serious and moderate injury. She told the jury there was no further legal instruction.

"I think they were very unsure as to what is the definition," said defense lawyer Chuck Hobbs. "That is a very serious legal term and it has been defined in other states and yet for whatever reason it's not defined in this one."

Hobbs said he will seek a retrial within 90 days.

Four Kappa Alpha Psi brothers were charged with using canes, boxing gloves and bare fists to beat aspiring fraternity member Marcus Jones, 20, of Decatur, Ga., so severely over four nights of an initiation ritual that he suffered a broken ear drum and needed surgery on his buttocks.

The fifth defendant was accused to assisting in the alleged hazing by encouraging Jones and other would-be fraternity members to bear up under the beatings and revived them with water when they passed out from the pain.

The definition of serious bodily harm has been an issued throughout the case. As the trial began nearly two weeks ago, Dekker denied a defense motion challenging the law as unconstitutionally vague because of that issue.

Her only instruction to the jury was it means injury that is neither slight nor moderate.

The defendants accused of striking Jones are Michael Morton, 23, of Fort Lauderdale; Brian Bowman, 23, of Oakland, Calif.; Cory Gray, 22, of Montgomery, Ala., and Marcus Hughes, 21, of Fort Lauderdale. Jason Harris, 25, of Jacksonville, was accused of assisting them.

"This was no accident. This was no joke. This was not playtime at the frat house," Assistant State Attorney Frank Allman said in his closing argument. "This was intentional. This was dangerous."

Allman bypassed reporters by leaving the courtroom through a back door after the mistrial was declared. A telephone message left with the state attorney's office after hours was not immediately returned.

Florida's 2005 hazing law makes it a third degree felony, with penalties ranging from probation to five years in prison. It also makes it a misdemeanor with a maximum penalty of a year in jail to create a risk of death or serious bodily injury even if none occurs. The defendants, though, were charged only with the felony, so the jury could not convict them of the lesser crime.

The final defense witness Monday, Dr. Kenneth Lee, an internist from Palm Beach County, testified that the bruising to Jones' buttocks was not serious and the surgery was unnecessary based on his review of photos and medical records.

"This is the type of thing I see in my office all the time," Lee said, adding that just two days earlier he had seen a 90-year-old woman with a similar but more serious injury due to a fall.

Lee said Jones' injury would have healed fine by using ice packs to hold down the swelling without surgery that left a scar. A small patch of dead skin removed during the operation would have fallen off on its own and his body eventually would have absorbed blood trapped under the skin, he said.

Allman relied on earlier testimony by Dr. David Fern, a surgeon who operated on Jones in suburban Atlanta.

Fern initially said the injury was as serious as anything he had seen in an auto accident but later acknowledged there were no broken bones, muscle or nerve damage, blood clots or lasting effects other than a scar.

The university has suspended the defendants pending the outcome of the criminal case. It also has suspended the fraternity until 2013 regardless of the outcome.