Reptilian means relating to reptiles or having the characteristics of reptiles. Reptiles are cold-blooded animals with scaly skin, like snakes, lizards, turtles, and crocodiles. When scientists describe something as reptilian, they mean it resembles or belongs to this ancient group of animals that has existed for over 300 million years.

You might see the word used to describe physical features: a reptilian tail, reptilian scales, or reptilian eyes that don’t blink much. The word can also describe movements or behaviors that remind people of reptiles. A character in a story might have a reptilian way of moving, meaning they slide or glide smoothly like a snake, or they might have a reptilian stare, meaning their gaze is cold and unblinking.

Sometimes people use reptilian to describe a part of the human brain called the brain stem, which controls basic survival functions like breathing and heart rate. This part of our brain developed early in evolutionary history and is sometimes compared to the brains of reptiles, which is why it has this nickname.