A shadow is the dark shape that appears when something blocks light. When you stand in sunlight, your body stops the light from reaching the ground behind you, creating your shadow. Shadows change size and direction throughout the day as the sun moves across the sky: long and stretched out in early morning, short at noon when the sun is directly overhead, then long again as evening approaches.

Shadows appear indoors too. A lamp creates shadows indoors, and even the tiny light on your phone can cast shadows in a dark room. The sharper and brighter the light source, the more defined the shadow becomes. Scientists and engineers use shadows to figure out the position and movement of the sun, to tell time with sundials, and even to detect planets orbiting distant stars.

The word also describes following someone closely, like a detective shadowing a suspect to learn their habits. When you shadow a veterinarian for a day to learn about their job, you're following them around to see what they do. A shadow of doubt means a small, lingering uncertainty about something. And when someone lives in another person's shadow, they struggle to get recognition because someone else is more famous or successful.