A tomboy is a girl or young woman who generally expresses masculine traits. Such traits may include wearing androgynous or unfeminine clothing and engaging in activities and behaviors traditionally associated with boys or men.

The word "tomboy" is a compound word which combines "tom" with "boy". Though this word is now used to refer to "boy-like girls", the etymology suggests the meaning of tomboy has changed drastically over time.

In 1533, according to the Oxford Dictionary of English, "tomboy" was used to mean a "rude, boisterous or forward boy". By the 1570s, however, "tomboy” had taken on the meaning of a "bold or immodest woman", finally, in the late 1590s and early 1600s, the term morphed into its current meaning: "a girl who behaves like a spirited or boisterous boy; a wild romping girl."

Before the mid-19th century, femininity was equated with emotional fragility, physical vulnerability, hesitation, and domestic submissiveness, commonly known as the "Cult of True Womanhood". Under the influence of this ideal of femininity, women did not engage in strenuous sports or any physical activity. This paradigm remained stagnant until the mid-nineteenth century. During the Long Depression of the late 1800s, the US's increasing economic instability made fragile femininity no longer desirable. Young women joined the workforce to support their families and learn practical job skills, and thus a more robust physique was needed to support the physical demands of job practices. This led to the paradigm shift in people's expectations of young women from languishing, decorative beauty to vigorously healthy, thus laying the groundwork for tomboyism.

In Charlotte Perkins Gilman's 1898 book, Women and Economics, the author lauds the health benefits of being a tomboy, that girls should be "not feminine till it is time to be". Joseph Lee, a playground advocate, wrote in 1915 that a "tomboy phase" was crucial to physical development of young girls between the ages of 8 and 13. Coupled with the birth of first wave feminism and the US's depressed economy, tomboyism amongst young girls emerged because the young girls' parents permitted or even promoted the tomboy upbringing due to the decaying economy and the American turbulent political climate.

During the American Civil War, American society fully realized the importance of healthy women. When hostilities of the North and South broke out and thousands of men fled to the battlefield, many adolescent girls and young women were pushed to be responsible for tasks that would be traditionally considered in the men's realm. Women who had not been allowed to have independent bank accounts were now expected to take care of the finances. American wives, mothers, and young girls who used to rely on the men in the household for security now had the duty of protecting their homes from the enemy. As a result, mothers focused on improving the physical constitution of their daughters while taking care of their own. Many women who had subscribed to the Cult of True Womanhood before the Civil War found themselves engaging in an array of masculine actions during it. Women being given the duties of men during the period of Civil War encouraged tomboyism.

While first wave feminism mainly focused on women's suffrage, second wave feminism expanded the discussion of gender inequality in areas such as sexuality, family dynamics, workspace, and laws in relation to patriarchy and culture. With the main purpose of critiquing the patriarchal system, this movement opened avenues for women in education, employment, and legal protection against domestic violence.

In the late 20th century, the term tomboy describes girls who wear unfeminine clothing, actively engage in physical sports, and embrace what are often known as "boy toys" such as cars, or other activities usually associated with boys. The term is used less frequently than before in the West mainly because it is now a societal norm for adolescent girls to engage in physical activities, play with peers of the same and opposite gender, and wear comfortable clothing.