Illustration have a long history, it has been in existence for centuries, but only been recognised as a distinct discipline fairly recently.
It is often confused with other disciplines most notably that of graphic design and fine art, perhaps because there is sometimes an overlap.
However: illustration is about communicating a specific contextualised message to an audience. It is rooted in an objective need to fulfil a particular task.
Illustrators need to have knowledge, understanding and insight regarding the context within which they are working, the subject matter, and work within the parameters and needs of the target audience.
They need to utilise a breath of intellectual and practical skills that might once be regarded as transferable from other disciplines but considered essential of the illustrator has ambitions to transcend the role of commissioned “hack”.
Source: Illustration, a theoretical and contextual perspective, by Alan Male