What is the main feature of vestigial organs?

Vestigial organs are features in organisms that have lost most or all of their original function through the course of evolution. The main characteristic of these organs is that they no longer provide a significant benefit to the organism's survival or reproduction, which is why they can be classified as serving no useful function. Common examples include the human appendix or the remnants of pelvic bones in whales and snakes, which illustrate how certain body parts can diminish in functionality when they are not under selection pressure.

In contrast, fully functional organs carry out essential tasks that are crucial for the organism's well-being, while those with a basic function still retain some utility in the organism's life. Essential organs are necessary for survival, making the distinction clear that vestigial organs do not fit into any of those categories. Therefore, vestigial organs are best described by their lack of significant function in the context of the organism's evolutionary journey.