What is the difference between rosacea and a rash?

Pustules. Rosacea is on the face has blood vessels redness and posible pustules, typically there is no scaling or dryness.
Specific vs general. A rash can mean any skin lesion of any description and from any cause. Rosacea is a disease condition manifested mostly with reddish cheeks (with or without pimple-like lesion), bulbous nose, and/or itchy eyelids. Every facial flush tends to increase the inflammation . However rosacea can also be called a rash.
Different causes. Rosacea results from stimulation of small nerves. Rashes arise in the skin due to allergies, viruses, or inflammatory reactions.