The recruitment, harboring, transportation, provision, or obtaining of a person for the purpose of commercial sex acts.
The most commonly used is from the Trafficking Victims Protection Act (TVPA). This was a cool thing that was put together to make sure trafficked victims were protected by the federal government. This is how the TVPA defines it:
Something that is done in advance to prepare for something else.
An important thing to understand is the difference between adults and minors in these situations. United States' law claims that anyone under the age of 18 legally cannot consent. We see this usually applied to sexual encounters between a legal minor and an adult. It is considered statutory rape because the law sees the minor as unable to consent to sex, and therefore a victim of sexual abuse by the adult. Minors also cannot consent to a contract. Prostitution of a minor includes both sexual encounters with adults and a contract. This means that legally, a minor is always considered a sex trafficking victim if engaging in any form of commercial sex. Force, fraud, or coercion does not need to be proved for the minor to be a victim.
An adult, however, is legally capable of consenting to contracts and sexual encounters. This means that unless it can be proven in court that the adult engaged in commercial sex because of force, fraud, or coercion, that adult is NOT viewed as a victim.
But what is commercial sex?
Commercial Sex: any sex act on account of which anything of value is given to or received by any person.
This includes: prostitution, stripping, and pornography.
In conclusion, when anyone engages in these things under force, fraud, coercion, or if the individual is a minor, they are a victim of sex trafficking.
The most simple definition we can come up with?
Definitions: dictionary.com and love146.org