Though I learned the common name for Campanula rotundifolia as bluebell, harebell is far more often used. This helps distinguish it from species of Hyacinthoides, in the Hyacinthaceae. On the other hand, the USDA PLANTS database uses bluebell bellflower, again demonstrating the variable nature of common names.
The distinctive bell shape of the flower in the genus Campanula (illustrated in these photographs from the Burke Museum: Campanula rotundifolia) earns its own morphological term: campanulate, or bell-shaped. Do note that the use of this term isn’t restricted to members of this family.