Stroke has several different meanings:

1. A single movement of your arm or hand, especially one that repeats as part of a pattern. When you swim, each pull through the water is a stroke. Different swimming styles like freestyle, backstroke, and breaststroke are named for their distinctive arm movements. When you paint, each sweep of the brush across a canvas is a brush stroke. A skilled artist might create a whole tree with just a few careful strokes.
2. A medical emergency that happens when blood flow to part of the brain gets blocked or when a blood vessel in the brain bursts. During a stroke, brain cells begin to die because they're not getting the oxygen they need. This is why doctors say to call 911 immediately if someone suddenly can't speak clearly, has a drooping face, or can't lift both arms. Quick treatment can save someone's life and prevent serious damage.
3. A lucky occurrence or moment of inspiration. A stroke of luck means something fortunate happened by chance, like finding your lost jacket right before a cold snap. A stroke of genius describes a brilliantly creative idea that solves a difficult problem in an unexpected way.
4. To move your hand gently across something, especially to pet an animal. You might stroke a cat's soft fur or stroke your dog's head while sitting together.