an artist who makes illustrations (for books or magazines or advertisements etc.)
"The illustrator spent hours sketching vibrant characters to appear on the pages of the new children's book."
a person who draws pictures (especially illustrations in books or magazines)
"The talented illustrator spent hours sketching colorful scenes for the new children's book."
In plain English: An illustrator is someone who creates drawings to go along with books, websites, or other things.
"The illustrator drew funny cartoons for children's books."
Usage: An illustrator creates visual artwork to accompany text, such as drawings for children's books or magazine articles. Do not confuse this role with an architect or designer, whose primary focus is structural planning rather than narrative imagery.
The word illustrator comes from the verb to illustrate combined with the suffix -or, which denotes a person who performs an action. It refers to someone who makes something clear or provides pictures for a text.