A wallflower is a person who is shy and introverted, often preferring to stay on the sidelines of social events. The term 'wallflower' can be used to describe two different plants: Cheiranthus, which is a genus of flowering plants in the mustard family, and Shrinking Violet, which is a type of wildflower native to Europe. Both plants are known for their small size and delicate beauty, making them an apt metaphor for someone who is shy and unassuming.