"Assault weapon" is also often used interchangeably to describe fully automatic firearms, such as machine guns. Automatic weapons keep firing as long as you hold a trigger. These guns are legal but tightly regulated.
Nearly every gun in common use -- from Lanza's rifle to the firearms novices use at their first shooting lesson -- is semi-automatic: You pull the trigger, one shot is fired, another bullet reloads in the chamber.
Magazine capacity is another component that can be -- but isn't always -- used to define assault weapons. The more rounds you can fit in a magazine, the less frequently you have to reload.
Legally, the meaning varies. The 1994 federal ban on "certain semi-automatic assault weapons" that expired in 2004 defined an assault weapon as a rifle or pistol that had a detachable magazine and at least two other specific, military-style features. Examples of such features are bayonet mounts, flash suppressors, pistol grips, grenade launchers or a weight of 50 or more ounces unloaded. That ban also prohibited magazines that could hold more than 10 rounds.
Laws regulating assault weapons, including the federal measure, also typically designate some specific gun models as assault weapons.
Some states have assault-weapon bans, but the definition of "assault weapon" in those measures varies.
California, for instance, bans semiautomatic guns if they have a detachable magazine and just one other combat-style feature. Connecticut's ban, like the expired federal law, prohibits firearms with detachable magazines and two such features. But Connecticut's definition doesn't restrict magazine capacity. That means the gun Lanza used, which held 30-round magazines, was legal.
Pennsylvania doesn't have any measure banning assault weapons.
New Jersey largely prohibits the weapons; people can purchase them, but only with a specific license that must be approved by police and prosecutors. The state's law defines "assault weapons" as firearms with detachable magazines and at least two military features. Also included are any guns "substantially identical" to any of the firearms specifically designated as assault weapons in the law, any "part or combination of parts designed or intended to convert a firearm into an assault firearm" and rifle magazines holding more than 15 rounds.
It's also worth considering what definitions of "assault weapon" don't cover.
Eugene Volokh, a UCLA law professor, notes on his blog The Volokh Conspiracy, that the bans on assault weapons don't address many of the features that influence how deadly a gun is, such as longer barrels, caliber or type of bullet. Some of the military-style features that assault weapons bans have focused on don't necessarily make the guns more powerful.
For more information:
The text of the expired assault weapons ban: http://www.gpo.gov/fdsys/pkg/BILLS-103hr3355enr/pdf/BILLS-103hr3355enr.pdf
Poynter's guide for journalists reporting on guns and gun control: http://www.poynter.org/latest-news/als-morning-meeting/182059/what-journalists-need-to-know-about-guns-and-gun-control/
Contact Emily Babay at 215-854-2153 or firstname.lastname@example.org. Follow her on Twitter @emilybabay.