President Barack Obama has famously declared that "human trafficking" is "modern-day slavery." He's also said that it "is a crime that can take many forms."

The second definition is a good deal more accurate. "Trafficking," in practice, is less a clear-cut crime than a call to moral panic. The vagueness of the definition allows or even encourages governments, organizations, and researchers to claim that there are tens of millions of trafficking victims worldwide on the basis of little more than hyperbolic guesses. Politicians use trafficking rhetoric to portray themselves as defenders of the downtrodden, and generate laudatory press coverage, as Cook County Sheriff Tom Dart has done with his crusade against Backpage.com and other sites advertising adult services. And some high profile figures have used trafficking narratives to gain fame. Somali Mam, the celebrated Cambodian anti-trafficking advocate, was exposed for making fraudulent claims about herself and other women she helped.

The exact origin of the term "sex trafficking" is unclear, but according to Alison Bass, author of Getting Screwed: Sex Workers and the Law, it seems to have been developed by anti-prostitution feminists in the 1990s. Bass told me that "trafficking" was used especially to describe the migration of women from the collapsing Soviet Union to the United States. Donna Hughes's seminal 2000 article "The Natasha Trade" defined trafficking specifically as "any practice that involves moving people within and across local or national borders for the purpose of sexual exploitation."

But anti-prostitution activists like Hughes often use “sexual exploitation” to include any kind of prostitution or sex work—in fact, Hughes insists in her article that "trafficking occurs even if the woman consents.” In other words, trafficking can include sex workers who decide to illegally or semi-legally migrate from Eastern Europe to the United States. This describes the majority of women who were said to be "trafficked," according to researchers Robert M. Fuffington and Donna J. Guy. "More often than not," they write in A Global History of Sexuality, "these women have engaged in some form of sex work in their home countries and see work abroad as a chance to improve their circumstances."

While Hughes defines trafficking as "sexual exploitation,” Obama also uses the term to refer to children pressed into military service and agricultural laborers forced to work under poor conditions or without pay. This definition has sometimes been endorsed by pro–sex worker rights organizations, who "hoped to redirect what had historically been a repressive anti-prostitution approach to an approach that regarded the sex sector as one of many labor sectors," Carol Leigh, a sex worker rights activist and filmmaker, told me.