Hostile means unfriendly, antagonistic, or opposed to something in an aggressive way. A hostile person acts with anger or opposition rather than kindness or cooperation. If someone gives you a hostile look, their expression shows clear unfriendliness or even aggression, like they're ready for a fight.

You can recognize hostility in someone's tone of voice, their body language, or their words. When a new student joins your class and another student responds with hostile comments, they're being unwelcoming and mean rather than helpful. A hostile takeover in business happens when one company forces its way into controlling another company against that company's wishes.

The word also describes environments or conditions that make survival difficult. Antarctica is a hostile environment for most living things because of its extreme cold. Space is hostile to human life without protective equipment. A hostile witness in court is someone who shows opposition to the side that called them to testify.

Hostility (the noun form) means the unfriendly or aggressive behavior itself. When hostility breaks out between two groups, they've stopped cooperating and started actively opposing each other. Understanding when someone's being hostile helps you decide whether to try talking things out, get help from an adult, or simply keep your distance.