A gravity knife is a type of folding knife that uses the force of gravity to release and lock its blade in place. It's often used by hunters, fishermen and outdoor enthusiasts. Other synonyms for a gravity knife include switchblade, flick knife, automatic knife, and push-button knife. These knives are generally prohibited in most US states due to their potential for unintentional harm. Despite the crackdown on their usage, these knives are still widely used in some professions. It's important to understand the legal implications of carrying a gravity knife and to use them responsibly. Always check local laws before carrying one.