Definition from Wikipedia
A sex toy is an object or device that is primarily used to facilitate human sexual pleasure. The most popular sex toys are designed to resemble human genitals and may be vibrating or non-vibrating. The term can also include BDSM apparatus and sex furniture such as slings, however it is not applied to items such as birth control, pornography, or condoms.
Alternative expressions include adult toy and marital aid, although "marital aid" has a broader sense and is applied to drugs and herbs marketed to supposedly enhance or prolong sex.
Definition from About.com
A textbook definition of sex toys usually describes them as any type of object or device that is used to enhance sexual pleasure. Outside of a research enterprise, there is little reason to insist on a universally agreed upon definition of sex toys. The definition I work from is that sex toys are anything that are not part of ones body, and that are used during some kind of sex play.
There are several legal definitions of sex toys, and across the United States, sex toys definitions can be found in both state and municipal law. In Alabama, one state that has restrictive sex toy laws, sex toys are defined as:
"device[s] designed or marketed as useful primarily for the stimulation of human genital organs for any thing of pecuniary value"
Other state definitions differ, but the primary elements are that a sex toy is designed to stimulate the genitals, and arouse sexual interest.
In terms of both commercial and public health definitions, sex toys are not considered medical devices, and are not regulated by any government oversight. For business purposes they are considered "novelty items" and are manufactured, distributed, and sold as such. This leaves many grey areas when it comes to legal liability of sex toys.