Trying to
define what a cat
toy is can be very difficult.
Why?
Because cats have a tendency to find something to play with just about anywhere, involving just about anything, at just about any time. Almost anything in the house can become, with the right situation, a cat toy.
Anything that dangles, swings, bounces, quivers, or just generally attracts the attention of a feline can suddently become perfect for being stalked and pounced upon. Anything loose, no matter what, can suddenly attract those little eyes, and soon finds itself being pushed and prodded by those paws.
Even the most mundane object, one that the cat has ignored for a long time, can become of extreme interest should it be mostly hidden, whether just barely sticking out from under something, or just the slighest bit visible through a crack. In any of these cases, it seems that instinct can grab a cat, and suddenly they're attacking this normally uninteresting object.
Beware of spending money at the store to buy something advertised as a "cat toy" - they are often no more interesting than random household objects, and sometimes appear to be even less so. The plastic ring from a jug of milk often leads to more playtime than a ball with a little bell in it. Your shoelace when you're standing in the kitchen may become more entertaining than that $5 toy sitting off to the side of the room, unused. Even your finger or toe, when wiggling just under a blanket, can become the most interesting thing they can find at the moment.