According to designer Jesse Schell in The Art of Game Design, a toy is simply "an object that you play with" (Schell 26). 

It is different from a game in that it contains no rules or mechanics within itself. However, toys can be essential to good gameplay, and include everything from avatars to weapons (90). Often it can be beneficial to build a game around the fun that comes from a toy, like the case of the Portal Gun in Valve's Portal.