The words "radical" and "radicalization" have been used by law enforcement and the media in describing Syed Farook and Tashfeen Malik, who killed 14 people at a holiday party in San Bernardino, California. But different people have different takes on what those words mean, and other terms are coming into use:

RADICALS AND HISTORY: Over the years, the term "radical" has been applied in the U.S. to a multitude of causes and people, including union organizers, feminists, suffragists, anti-war activists, communists, anarchists, civil rights leaders and even Republican lawmakers who wanted the South dealt with sternly in the aftermath of the Civil War.

RADICALS AND THE FBI: The FBI's J. Edgar Hoover used "radical" at nearly every turn during the Cold War, the civil rights movement and the Vietnam era. The modern-day FBI employs the term sparingly and considers the embrace of violence an essential component of "radicalization." The bureau's website defines it this way: "The process by which individuals come to believe their engagement in or facilitation of non-state violence to achieve social and political change is necessary and justified."

RADICALS AND HOMELAND SECURITY: The U.S. Homeland Security Department steers clear of "radical," instead using "violent extremist" and "violent extremism." The agency defines violent extremists as "individuals who support or commit ideologically motivated violence to further political goals."