Herida vs. lesión
While both 'herida' and 'lesión' can be used in reference to an injury or wound in Spanish, they are typically used in different contexts and have subtle differences in connotation.
Herida
'Herida' generally refers to a physical wound typically caused by an external factor, often involving a break of skin or tissue.
Ella tenía una herida profunda en el brazo.
(She had a deep wound on her arm.)
La herida está sangrando mucho. Necesita atención médica.
(The wound is bleeding heavily. It needs medical attention.)
Lesión
'Lesión', on the other hand, is commonly used in medical or sports related contexts to refer to an injury often resulting from strain, overuse, or damage to the body's tissues without necessarily breaking the skin.
Él sufrió una lesión severa en la rodilla durante el partido de fútbol.
(He suffered a severe knee injury during the soccer match.)
Esta lesión requerirá rehabilitación para que pueda volver a caminar normalmente.
(This injury will require rehabilitation in order for him to be able to walk normally again.)
Summary
'Herida', usually implies an open wound. It implies that there has been an external breach of the bodily surfaces and is commonly used in general cases. On the contrary, 'lesión', while still signaling an injury, suggests damage beneath the surface of the skin like sprains or torn muscles without necessarily implying that there are external wounds. It is used more frequently in medical or sports related disciplines.