A constant is something that stays the same and doesn't change. In math class, you might work with constants like the number 5 in an equation, which always equals exactly 5 no matter what. Scientists rely on constants like the speed of light, which never varies. When something remains constant, it's steady and reliable.

The word also describes a person's behavior or attitude. A constant friend is someone who stays loyal through good times and bad, whether it's easy or difficult. You might have a constant practice routine, showing up every day to work on your instrument or sport. When a dripping faucet makes a constant noise, it means the sound continues without stopping.

The opposite of constant is variable (which changes) or occasional (which happens sometimes but not always).