Definition

From Wikipedia: 

Shōjo, shojo, or shoujo manga (少女漫画 shōjo manga?) is manga marketed to a female audience roughly between the ages of 10 and 18. The name romanizes the Japanese 少女 (shōjo), literally "little girl". Shōjo manga covers many subjects in a variety of narrative and graphic styles, from historical drama to science fiction — often with a strong focus on human and romantic relationships and emotions.

From Manga Updates: 

A work intended and primarily written for females. Usually involves a lot of romance and strong character development.